Why do you want a sub region?

The CH 17 tutorial uses an impeller blade (a thin surface between the solid and blade) to create an interface for the impeller. IDK, I'm just trying to mimic the tutorial. Is there a way to do this with only using lids and an inlet/outlet boundary condition? I don't want permanent boundary condition values because I want the "fan" to do the work.

Also do the flow in an axial rotor/stator simulation. That will show a different approach to rotating machinery modelling.

Don't forget that if you use a momentum source approach (ie do not model the fan but only its momentum added to the flow) you can define the source term to follow the fan performance curve. This means you are not using permanent values and the fan does the "work".