This is a difficult question to answer, for several reasons, but the biggest problem is that we don't have enough information to form a conclusive answer. However, we can make some informed guesses.
It goes without saying that dragons are greedy, selfish creatures, and care about gold, jewels, and other material riches more than anything else. This suggests that Smaug might be open to an alliance if he was promised treasure. It appears that Sauron had access to such things, as a passage from The Two Towers makes clear. While Merry and Pippin are captives of a group of Orcs, one orc suggests that they should be handed over to a nearby winged Nazgûl. Another orc, suspicious of the first, says:
"Maybe! Then you'll fly off with our prisoners, and get all the pay and praise in Lugburz..." [Note: "Lugburz" is Barad-dûr, Sauron's tower]
-The Lord of the Rings, The Two Towers, Book III, Chapter 3: "The Uruk-Hai"
This obviously means that Sauron pays his soldiers, probably in booty. Of course, if he was successful in his war against the west, he would have had access to even more treasure. This alone might be enough to persuade Smaug to join Sauron.
But how would Sauron contact Smaug? I don't think it would be as difficult as you suggest.
Send Orcs: I don't think we should dismiss this idea out of hand. There is no reason to believe that Smaug would kill any Orcs who approached him by default. He didn't kill Bilbo upon their first encounter, and Gandalf says in Unfinished Tales that he selected Bilbo for the mission because Smaug would recognize Dwarves by smell, but he had never smelled a hobbit before. He would presumably be able to distinguish between Orcs and Dwarves by smell as well. And it is important to note that The Silmarillion implies that dragons were one of Morgoth's many terrible creations; Sauron, as Morgoth's most powerful and trusted lieutenant, could easily assert his right to command the loyalty of his former boss' servants, including the dragons. If the Orcs were acting in the capacity of Sauron's emissaries, it would be unwise for Smaug to kill them outright. Granted, Sauron doesn't show much reluctance to kill his Orcs, but he probably wouldn't like it if someone else killed them, especially when they were sent on a special mission by Sauron himself. And if the Orc delegation said "We're here to offer you a crap ton of gold", Smaug would probably be more than willing to hear them out. The best comparison to Smaug encountering a delegation of Orcs is the clear coexistence of the Orcs of Moria with the Balrog known as "Durin's Bane". The Balrog was hostile to the Fellowship of the Ring, but he appears to have left the Orcs more or less alone, and the answers to this question suggest that this was because they served the same master. The obvious lack of verbal finesse we see in the Orcs wouldn't necessarily be a problem, since they would only have to say "Sauron sent us and he wants to offer you a whole bunch of booty!" before Smaug turned on the flamethrowers.
Send himself: This is almost certainly out of the question. Sauron very rarely does anything for himself. Although it is possible: During the quest for Erebor (i.e., the events of The Hobbit), Sauron wasn't in Mordor- he was in Mirkwood, in Dol Guldur. This puts him only a few hundred miles away from Erebor. When Gandalf and the White Council investigated Dol Guldur, and Sauron fled, he could have stopped by the Lonely Mountain and said hello to Smaug. However, it doesn't seem very likely.
Send the Nazgûl: This strikes me as the most plausible option. The Nazgûl are afraid of fire, but their presence is felt long before they are near, so Smaug would not mistake them for anyone else. We have no reason to believe that Smaug could kill them, even if he tried. In Unfinished Tales, we hear the reasons Sauron is reluctant to use the Nazgûl in his hunt for the Ring: Except for the Witch King, they tended to wander off when they were sent out alone; except for the Witch King, they were afraid to cross water without a bridge or shallow ford; they were almost incapable of traveling without being noticed, because of the terror they inspired among everyone in their path. With the exception of their fear of water, none of these factors would have been a problem if they were sent from either Mordor or Dol Guldur to Erebor. The geography was such that they could reach Erebor without passing by any settlements, and they would have had to cross only one, or at most two rivers. They only wandered off if they were not accompanied by the Witch King, so if Sauron put him in charge, they wouldn't stray from their course. I see no reason why the Nazgûl couldn't have gone to Smaug on Sauron's behalf.
Send the Mouth of Sauron: You didn't mention this possibility, which I think is a very good one. The Mouth of Sauron is more competent, capable, and, well, diplomatic, than the Nazgûl are. They seem to prefer shrieking like Banshees to actual speech; The Mouth of Sauron is positively eloquent in comparison. He appears to be a trained diplomat, so much so that he loudly objects to being stared down in a menacing way by Aragorn, reminding everyone present that it is against the rules of diplomacy and warfare to attack an emissary sent to parley. He would be entirely capable of conducting business with Smaug on Sauron's behalf.
Other means: There are any number of other options Sauron could employ if he wanted to talk with Smaug. We don't know what they are, but he obviously has tremendous power and can control his servants at a distance. When he realizes that Frodo and the Ring are inside of Mount Doom, his entire army is thrown into disarray because all his attention is redirected to the volcano, so they are no longer being guided by his will. This creates the possibility that he wouldn't need to send a physical delegation to Smaug - he might be able to contact him through some form of telepathy. Sauron is a magical being, and it is difficult to categorically reject any possible use of his magical powers to further his goals. Unfortunately, we don't know what was and wasn't within the limits of his powers.
Obviously, since Sauron never contacted Smaug as far as we know, all of this is speculation, but it would probably be relatively easy for him to do so, in any number of ways. The distance from Mordor to Erebor is considerably less than the distance from Mordor to the Shire; the distance between Dol Guldur and Erebor is far shorter.
