Propeller simulation in "moving flow"

Mark, I'm not sure what you mean by "because it slows down the propeller to match initial speed". The flow can't move the propeller, you have to set an angular velocity when you do a rotating simulation. There is no coupled fluid-structure interaction in this software.

As for your original question, you can use the initial boundary conditions in the project setup to set an arbitrary velocity in any of the 3 primary direction vectors.