This could be of some interest: https://www.youtube.com/watch?v=5EV1elNFNSM
Once you have done, just export it in OBJ format and import it in SweetHome3D.

There is this too: https://touchterrain.geol.iastate.edu/
It allows to export to OBJ format, but SH3D fails to open it, maybe it's just matter of adjusting some lines in the file (missing material?).