AFAIK, there is an option to activate direct input mode in Japanese IME. Here is the steps to do so (using Windows 7 as sample):
a. Using language bar
Click the drop down button on bottom-right corner of language bar while Japanese IME is being active, click "Properties" to add the menu, then click on it when appears.
![IME Properties Menu](https://cdn.statically.io/img/i.sstatic.net/EFa1d.png)
When dialog box "Properties for Microsoft IME" appears, go to "Editing" tab, then uncheck "Don't use direct input mode" (as shown in supplementary image).
![IME Properties Editing tab](https://cdn.statically.io/img/i.sstatic.net/Zlu9e.png)
b. Using "Region & Language Options"
Go to Control Panel => Region & Language => Keyboards & Languages tab => click "Change keyboards" button. The "Text Services & Input Languages" dialog box will appear as shown below:
![enter image description here](https://cdn.statically.io/img/i.sstatic.net/4GG4H.png)
Select "Japanese - Microsoft IME" & click "Properties" button, then "Properties for Microsoft IME" dialog box will appear and follow same procedure above to uncheck "Don't use direct input mode" option.
NB: Don't forget to apply changes using "Apply" then "OK" button and restart all Office applications which exhibit that small input popup box.
Related issue:
Weirdness with Japanese IME and MS Outlook (also apply for other MS Office programs)