It is a limitation of Wave ports.
But I think you can use Lumped ports anywhere inside the geometry. More explanation:
Wave ports solve actual field distributions for one or more propagating or non-propagating Modes.
Lumped ports excite a simplified, single-mode field excitation assuming a user-supplied Zo for S-parameter referencing.
Limitation of lumped ports:
Lumped ports excite only one mode, and therefore are not appropriate for excitations where modal superposition is expected.
Since impedance is supplied by the user, not computed, no alternate definitions (Zpi, Zpv, Zvi) are supplied.
More information can be studied in the attached file.
good luck!