The Auto Face Extruder allows you to automatically extrude or bevel out selected faces in the scene into a box or a pyramidal prism which allows for the easy conversion into Source Engine thereafter.
The script also has an option to limit the selected polygonal faces to tris if the selected faces have more, which gives better results when using the bevel option since invalid solids are more common with those when converting to the source engine.
This tool isn't particularly effecient, it may take over an hour to convert a complex (tens of thousands of tris) scene, since it only uses MAXScript instead of the SDK, but it is useful in some cases.
Tools to convert to Source Engine after the tool's conversion:
Wall Worm Model Tools , CannonFodder's SMD Tools , Wunderboy's SMD Tools