Why would you want to play a cleric/thief?
It's probably the most useless combination of classes, since the cleric's weapons restrictions means you won't be able to backstab.
You can still backstab with clubs and quarterstaves. Staffs in particular are really excellent backstab weapons, due to how many really good ones there are.
For the original question, I don't know of any way to restructure ANY of the classes' action bars. They appear to be hardcoded. So unfortunately, no, all of the thieving has to be done out of the special abilities... but that's really just because cleric/thieves have too many abilities to all fit on the bar at once. Still, you can hit F12-F1 to access them, which might save some time vs. clicking.