Technically possible yes, but ACs devs don't what their assets ported to other games, so even if you did port to BeamNG the mod will likely receive a DMCA takedown As for AC mods, that is a case by case basis, depending on whether the mod author is willing to grant permission or not
If we are the mod creators, and own the IP in the mods for tracks, cars, karts, ambulance etc how easy is it to import to BeamNG?
Not all that hard I would imagine, provided you have some basic 3D knowledge I can't speak for tracks, but for cars even if you converted all the mesh/materials over, only half the work is done as now you also need to create a fitting jbeam (or adapt one form official content)