Answer:
First off I'm no expert and am myself looking for the answer to this question - but from what I've heard on internet so far, the outlook isn;t that good.
If your music on sonic stage is in MP3 format it will be possible to transfer it to itunes, but if in the ATRAC format (used automatically by sonic stage if you rip CD's into it) then it is not possible - But I hae heard you can get an ATRAC MP3 CONVERTER, or another option (all be it labourious) is that you burn your traks back to CD from sonic stage and rip them back into i tunes.