1)Subscribe to addon that you want to get in sfm
2)Go to "%Steam\steamapps\workshop\content\4000\ (addonID) \ (addonID)_legacy.bin
3)Extract .bin file as archive (use 7zip or WinRAR )
4)Copy extracted file AND gmad.exe from GarrysMod\garrysmod\bin to desktop (or any folder)
5)Rename extracted file extension to .gma and drag-n-drop this file to gmad.exe
===At this point gmad.exe will create new folder with extracted addon files===
6)Get the mdl file and decompile it using Crowbar modding tool
7)In Compile tab, set "Game that has compiler" to SourceFilmmaker
8)Compile it!

Or using blender:
7)Download Blender Source tools
8)import .smd file in Blender and export it as .dmx

For High-poly meshes use bodygroups, to reduce the number of vertexes per model