Is it possible to combine Common and GND on a DC/DC converter?
The Traco TMS 2 series are isolated converters, so "ground" on the input side is, well, isolated from "common" on the output side.  Another company may have called them "input return" and "output return".

Yes, you can tie them together.  The two sides will no longer be isolated (in which case you may want to see if you can use a non-isolated module that costs less), but the thing should work just fine.