I've just built a Cardan Joint, and I have to add an extension to the rotating pin at the bottom. The thing is that the pin at the bottom (inside red circle) is a cylinder, and thus can't grab the extension (marked with a red X). Therefore I would like to add a small box (inside blue circle) at the end of the pin, so it can fit into and grab the extension, and make the whole Joint rotate.
But how do I add this inside the assembly? I tried sketching it onto the cylinder from the bottom and then make an extruded boss/base, but the whole joint didn't rotate. Any simple way to do this?