Is this possible?
With most games yes, but i think SF4 is protected so it won't work.
If i'm wrong, put a data stick into your PS3 (formatted to FAT32) and go to the same game folder. Find the savegame, press triangle and select copy. If i'm right it'll tell you to bugger off, if not it'll copy to the data stick. Then you can switch account and copy it back.
Tbh if it doesn't work it's possible to unlock all characters in an hour anyway.