I'll get a scene with whatever I want modeled, and save as a VMF like youre making an instance in hammer. Then import that vmf through wallworm model tools in 3dsmax, then export the objects as an fbx and bring that into Maya. Then export the fbx from maya back into 3dsmax to compile as a model to bring into source.
Also helps to get a scale of something you want in hammer to the units there, I made those modular ceiling pieces to scale from hammer brushes, (16x16, 64x64, 128x128)
Hammer Vmf -> 3dsMax Wall Worm Import -> Export as FBX to Maya -> Export as FBX to 3dsMax -> Import FBX and export QC model to Source