It's possible for sure. The main problem would be the lighting as GoldSrc models gets enlightened just by the model's origin. Even if the lighting is baked onto the textures, you'll still notice the black/dark zones in game on the model if a light entity points at the model's origin (which is something awful.)
So the best thing would be:
- Export the model as ".obj"
- Import it into Sledge as it has an .obj importing function
- Save it as .rmf
- Fix/rebuild those brushes you don't want to be triangulated or those ones that you don't want as tetrahedrons (to possibly decrease wpolys in game.)
I've made the opposite thing a while ago because I wanted to make a 3D skybox as a model. So I've basically built everything into Hammer and Sledge, compiled, opened the .bsp into Crafty, exported the whole map as .obj, imported into kHED, fixed some things and added Groups and Smoothing, compiled as .mdl, done.