It's been a while since I have done it but I have done this by saving a .3ds file and then opening that in Rhino and then saving it as the .sdl format. I actually didn't have Rhino so the the lab converted it for me. Worked great, but it was a few years ago so I'm not sure if that method is still viable.
matt johnson
archicad since 2004
imac 27, 4.2 GHz Intel i7, 16 GB RAM, radeon 575 4 GB, macOS 10.14.6
imac, 4.0ghz i7, 16gb ram, Radeon M9 390 2GB, OS X El Capitan