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.