Is it possible to use the same ground reference for multiple single or internal ports? Some layouts I am designing have a finite ground plane with ground reference ports underneath several metal structures, each of which have single ports attached to them and the finite ground plane is the common return path for all signals on the single ended ports. It is not possible to attach multiple ground reference ports to the ground plane in the same referance plane.
From the description of your design, it seems that you could just add one port to your finite groundplane (preferrably pick a point that is close to the location where in the real life structure, the finite groundplane is grounded) and set that up as a regular port (e.g. internal port).
After doing a Momentum simulation with this setup, you will of course end up with (n+1) S-parameter data, where n is the number of original ports without the one on the finite ground plane. To connect the extra port to ground, you need to go to the schematic and perform a circuit simulation. The easiest way is to use a Layout Component for this.
If you create a Layout Component for your layout design (Momentum>Component>Create/Update), you can go the schematic environment and connect the port you added to the finite ground to ground. This will force the return current through the port on the finite ground plane.
PS. You can use multiple ports on the finite groundplane as well and study the effect of imperfect grounding.