Wwise plays well with multichannel files set to 3D. Your control over how it will sound in context depends on how much Spread you apply over distance (in Game-Defined attenuation editor). You have to create a custom curve as by default, Spread is set to None.
BTW: In many cases, there is no need in changing the channel count outside of Wwise. You can trust the Conversion settings if you want to convert Stereo files to Mono (and I'd trust Wwise much more than many wave editors out there for this process, but especially for sample rate conversion).
Hope this helps
Simon