Yeah, it is very possible.
You'll have to write a .vmt (?) file that declares which texture to use and which properties it has.
I have done several custom wood props and just used the base wood textures.
The only difference between the model textures and the terrain textures are lighting declarations in the vmt file. So all the building objects actually use the same texture and the terrain textures you use in the editor, but they are vertex lit. Vertex lighting doesn't work on terrain and makes those tetxtures turn black or full bright depending on how you look at them.
There is a 'complete guide to importing custom models' tut in the tut section. Look at that for vmt info.
Oh, looks like 3d ryan has the link up there. /\