The best way to do this is to create a layout component of the Momentum design, include that in a schematic window and setup a new S-parameter simulation. From the schematic window you can tune the characteristic impedance of the port.
This will not require to redo the Momentum simulation.
I tried it before the way you suggest. It works but only if you have everything defined in one layer. The thing is that I have a multilayer structure. Can I define layout components for such a structure? It seems that I can not even define MLINs on 2 different layers. I can do "Insert Rectangle" command and insert rectangles on different layers, but not MLINs. But if I have only rectangles, how can I define them as layout components?
You can use layout components if you have multi-layer design. Just setup the design for multi-layer simulations with Momentum and create a layout component.
Can one vary separately real and imaginary part of the port impedance? When I imported my parametrized component in Schematic window, the port I defined in Layout window was lost. Then I defined a termination instead. Thus, impedance of the termination can be varied. By the way, when you say that it is possible to vary impedance of the port in Schematic window, do you mean "termination" instead of "port"?
This will not require to redo the Momentum simulation.