You can tune the ULX system to any frequency you want. But the PGX system is limited to certain groups and channels. It is possible but quite inflexible. You will also run into problems with the different compander and pilot tones within the differing units. With your PGX system you should have got a chart of frequencies which correlate to the frequencies. Match this up with a frequency on the ULX handheld and this should work ok. The "sync" feature will not work you will have to do this manually.
Answers & Comments
You can tune the ULX system to any frequency you want. But the PGX system is limited to certain groups and channels. It is possible but quite inflexible. You will also run into problems with the different compander and pilot tones within the differing units. With your PGX system you should have got a chart of frequencies which correlate to the frequencies. Match this up with a frequency on the ULX handheld and this should work ok. The "sync" feature will not work you will have to do this manually.