If you like to associate slot # with Ref #, just add Ref Suffix property to the slot and move the suffix next to Ref. You also can add a Ref Incr property on the block symbol to force it label Ref from a certain number.
Althoug there is no way to customize Ref pattern, but labeling ref is versatile.
I remember seeing a request a while ago for the type of reference designator that you are asking for. The reply at that time was that the reference designator is for the part. The individual gates of the part all have the same reference designator as the part. To see which gate/slot it is, you have to show the pin numbers.
I don't know if Mentor has changed the internal database format; nor has added funcionality to add a suffix to the ref-des to show the gate number since the original request, but I would be surprised.
The Expedition packager does not support reference designators sub-divided using a letter to differentiate each gate, all gates in a package must have the same reference designator.