Yeah it's pretty hard to walk inside the buildings in arma 2, because it was made to play most of the time outdoor at the inverse of SWAT. I think there is no mods to correct that.
And how do you place units within buildings in the editor?
You place your unit inside the bulding you want and in the init field you put this: This setPos [getPos This select 0, getPos This select 1, x]
Then you just have to replace the "x" by the height (in meters). the bad thing is you allways need to try a height then press preview to see where the guy is. if you put 10 he will fall from 10 meters to the roof. if you put 1 he will be in first floor.
I will check on armaholic, long time ago i found a script who put automatically peoples (civil or ennemies) inside buildings.