Is there a way to create our own controller voices using Sapi5 ? If so, we could create a bank of voice files & have a greater choice to use in Pilot2ATC.  Is this possible ?

It is technically possible, but you have to have your voice made professionally with a company like Cereproc. You cannot do it yourself and share it with other people. It's also likely expensive as they dont quote the price for this service on their website.