As long you define a ground reference, a lumped port can be defined anywhere in the geometry - interior or exterior. The following requirements must be satisfied in any lumped port application.
Every lumped port must have an ground reference, either by defining a differential port or by defining a explicit patch that connects the lumped port to a ground conductor.
Lumped ports cannot be defined on a wave port plane.
Lumped ports cannot be assigned to poor conductors. However, in case of an explicit ground reference, the material assigned to the patch solid is irrelevant. For details on what constitutes a "good conductor", see Materials.
A differential port must have exactly one positive terminal, though it may have any number of negative terminals.
Any symmetry planes in your geometry must bisect the lumped port and its patch. In the case of differential ports, the symmetry planes must bisect all terminals of the port.