[andrewas]

Nope. It cannot be done, the sorc class is hardcoded. You can create a new kind of sorc, like the Druidic sorc, but you cannot allow multi or dualclassing. Just one of those limitations that are built into the engine.

However, if Icewind Gate ever comes out, this will port the game to the IWD2 engine which uses 3rd ed and that allows multi-classing far more freely than the 2nd ed BG2 engine.