It's been a while after you asked the question, but for reference of others, in the transient solver you can choose "calculate modes only". After that, CST only solves for the waveguide port. Then, under 2D/3D Results, you can see the calculated modes for your port. Note that, to be able to talk about line impedances, your waveguiding structure needs to support TEM or Quaisi-TEM modes (microstrips, coaxial lines, striplines, slotlines etc.)
I don't quite understand why you need to use waveguide port though
I hope this is helpful