When you first create your party and equip them, before you do anything (and I mean anything) else, go to the transport mirror in Fountain Head, boot down the screen in front of it, and when it asks you where you want to go, type in "Doe Meister" (without the quotes). It will transport you to a secret cave within Dragon Cavern, where you can pick up (IIRC) 4 million gold. Then click on Mr Wizard to return you to Fountain Head. (This costs experience, which is why you do this before anything else - if you haven't got any experience, using Mr Wizard can't cost you anything.)Then, there's a lion statue in the Hall of Insanity where, if you pay the money demanded, it will confer all skills on the selected character. That includes the Thievery skill - it's the only way you give that skill to anyone other than a rogue or ninja. You will find your sorcerer makes a perfectly adequate thief after this treatment. 
Again if you know exactly what you're doing, there is also a way to use the 4 chairs in the Tomb of Terror with a party of about level 11. If you then use two chairs on the sorcerer and two on the cleric, and restore their stats (using each chair gives a character 25 levels and drops all his/her stats to 3) you really won't have any further thievery problems.
