ScriesM has done it, he's imported (Rather easily) a unique pokemon into SWSH
https://twitter.com/SciresM/status/1196342543425781760
It seems upscaled textures have already been done
Things left to do according to him:
Obvious action items:
-Produce flatbuffer schemas to add new animations.
-Produce converter from BCH to GFBMDL, to use USUM models (no converter exists right now).
-Upscale old pokemon textures. (Seems to have been done)
Someone tried to do imports using Blender, according to some, however, Gamefreak uses Maya, this might be the cause of the issue he has.
"As far as I've seen. Pokémon lets go models are the same than USUM (You can use same rigging and textures). However, SWSH meshes break when importing the rigging."