How do I create a library feature that I can drag a "prompted size" component into an assembly and it create a new part for me (virtual part ok too).  Structrual shapes in particular and not using the weldment tools.  I just want to drag a profile shape in the assy, define the length of the structural shape, and have it created as a new part in the assy.

I'm sure I can create new part in the assy and then drag a sketch into it.  Just wondering if there's an easier way.