We do not currently support MP2/MP3 and have no plans to do so in the short term. It is also not currently possible to write a user codec plug-in with the SDK. We fully support Vorbis on all platforms; it is a higher quality codec than both MP2 and MP3, with CPU usage similar to MP3.
However, you may decompress MP3 data on your side and push it into the Wwise sound engine by implementing an Audio Source plug-in. There is a sample in the SDK which does that (PC only), in <WWISESDK>/samples/Plugins/AkMP3Source/.