The Brawler hybrid class is proficient with all weapons in the "Close" Fighter Weapon Group, and can use multiple abilities, such as increasing the damage of these weapons, or using these weapons as part of a Flurry.
Is there any way to add additional weapons to this group, using only official Paizo material?
Yes, kind of.
Specifically, you can make any melee weapon part of the Close Weapon Group with the Weapon Modification called Versatile Design for 500g (mundane) or 750g (magically Enhanced).
When versatile design is added to a weapon, choose a fighter weapon group. The modified weapon is considered to be a weapon of that weapon group (such as for the fighter’s weapon training class feature).
Use with the Brawler's class features would be up to the GM, so check before you invest. However, it should work.
Note: Any weapon modification increases the 'difficulty' of the weapon by one- Simple becomes Martial, Martial becomes Exotic, and Exotic becomes impossible to wield properly. Furthermore, specific weapon proficiency (IE Rogues have proficiency with Rapiers) no longer applies. The Modified Weapon Proficiency and Weapon Adept feats (as well as Martial/Exotic Proficiency, where applicable) allow you to bypass this if it would cause problems with your ability to wield your weapon of choice.
