Hi, Ado:
On IE3D, the default reference plane is where you define the port. For single ended extension port, you can select PORT->DEFINE REFERENCE PLANE to define a shift of reference plane immediately after you define the extension port. For differential port, you may not be able to use such a feature, you may have to use the BACK SIMULATION feature on MODUA to shift the reference plane. A CPW port is a differential port. We may automate such a feature in the next release.
Best regards,