Why is the built-in File > Import > { ACAD | 3DS | DEM ] not sufficient? You want a Ruby script that automagically does the conversion for you? There are enough other, free, conversion utilities that I doubt any programmer of sufficient skill would consider that task to be worth their time.

Or do you just want a Ruby script that will, or instance, open Blender, load the file in the non-SU-importable format into Blender, save it from Blender as 3DS, and then import it into SU for you? I suspect, but I might be wrong, that this kind of manipulation of another program's GUI is beyond the scope of Ruby. If you're on Windows, you might look into AutoIt which will let you write scripts to open programs and control their GUIs. You might even be able to get SU to kick off that AutoIt script for you. I'm not a Mac person, but I bet there's a similar scripting utility for Macs.

Unless you have hundreds of files to convert on a daily basis, just cranking things through something like Blender to do the conversion ought to solve the basic problem of using files in a non-SU-importable format. If you have to do this dozens of times daily, then AutoIt might smooth that process for you. But the work involved of being able to do it transparently inside SU would demand that you have LOTS and LOTS of conversions to be worth the trouble involved.