I tried this too, doing a quick RNAV flight from KPAE to KBLI. On approach into KBLI runway 16, I'm able to capture the localizer using RNV/APR on the KNS-81, approach mode on the autopilot, and RNAV selected on the GPS-NAV control selector. But while the HSI displays glideslope indicators, the autopilot doesn't capture the GS. My assumption is that I'm doing something wrong, but I did notice one thing.
According to a KNS-81 manual I found online, the unit should recognize an ILS frequency and display no radial or distance information. What happens instead is that I can input both radial and distance data for an ILS frequency. Maybe the unit isn't properly recognizing ILS approaches? Or maybe I'm setting it up wrong. I'm not sure.