Thanks, gonk, I spoke with Steve at Outlaw tech support to see what he could tell me about interfacing to the Niles system which I already have in place. He said that the 794-10 should work as an interface between the 990 and the Niles system unit as well. He gave me the DIP switch positions to use (turns out to be same as Sherwood as listed in the 794-10 manual). Ordered one (plus power supply). I think the connection from the Niles base unit to the 794-10 should be compatible.

I'll report back once I get a chance to play with it.