Normally our FTN's have an Aerospace Systems Course. The NTPS and the PaxR. systems courses are also good.
As far as a Navigator going FTE, if you have the experience requirements, then apply, as others have. The flying background is definitely useful, both in completing the course and in the work afterwards.
These definitions (FTE, FTN, and even TP) are slowly becoming obscured, even more so when we consider much of the systems testing.