Unfortunately, it is not possible to give a character of one class an ability button from another class. For example, you can't give a Sorceror a Thieving button or a Turn Undead button. The classes are hard-coded into the game engine. That is also the reason why you cannot multi-class a Sorcerer (or a Paladin or a Bard).
However, there are creative ways to give a character new abilities. For example, if you want to play a Sorcerer/Thief, you can create a Mage/Thief and give him a special ability that lets you refresh his spells as often as you like, thus making him in effect a "Sorceror". For more discussion on this subject, check out
this old message thread from a couple of years ago.
Hide in Shadows is easy to duplicate with Invisibility spells or, better yet, with the Staff of the Magi. To open chests and doors, you can cheat and carry a bunch of Knock scrolls. To find traps, you could use an item editor to create a scroll based on the Find Traps spell (you can make it useable by any character). Although you didn't mention it, I think you can give any character the innate ability to Set Traps.
Stretching the rules can be tedious, but the alternative is to play the game the way it was intended.
Linear Mode
