You can create a .bsp and just set it as a model for an entity as you would with a .mdl. I think you have to set SOLID_BSP as well, and probably some other things like the proper movetype. The model will actually have proper bsp collision instead of using the bounding box. It may look out of place as the lighting is baked into the .bsp file, instead of being lit by the environment like normal models (may be fixable?). And if I remember correctly, you can't have any entities in the .bsp as it will actually overwrite entities in the map that's being played.
I don't remember many details but I do remember experimenting with this years ago and it kinda worked.
There's also a way to make .mdls use hitboxes for collision instead of a boundind box:
https://forums.alliedmods.net/showth...65#post2485265