Hello @luuxpuux! Welcome to the littleBits forum.
I don't know about 3D printing the part, but I made a connector for a Lego axle by taking an X-acto knife to the insides of the red, flexible lego axle connector. Once I removed the plastic inside (that made the cross shape of the axle) on one half of the connector, it slipped right on a standard pot (which the dimmer uses.) It was crude, but it worked.
If you succeed in making a 3D file, I'm sure it would be very popular here!