I don't know of any way to make a true multi-class Sorcerer. The classes are hard-coded into the game engine, so you have to find a "workaround" if you want to play a multi-class combo that the game does not recognize. For example, if you want to make a multi-class Kensai with another kit, the game does not allow the use of two kits, but you can give your character the Kensai special abilities with an editor and call him a "Kensai". For most people who want to play hacked characters, that's close enough.
The easiest way I know to mimic the abilities of a Sorcerer (so that you can call your character a "Sorceror" when that's not really what he is) is to give your (multi-class) Mage a spell that will allow him to rest at will. That way, he can re-memorize whichever spells you want him to cast without resting "properly", which will give him the ability to use any spell in his spellbook as many times a days as you like. I made a little spell that you can download if you look in
this thread.
Good luck!