"add a couple of vias for routing and pouring" - in Layout, select the net, and then <right-Click><add Via>
You can add a via in the design (i.e. Layout). You can't define a via for a part (i.e. Part/padstack editor). If you want a thru-hole pad (i.e. a via) in a part it needs to be defined as a terminal.
Note that everything I've said is in Layout.. are you trying to update your part/library in xDxD?