The Helixnet digital Power Line Modem (PLM) technology is not the same as the older, analog PL systems, so you CAN NOT connect the two systems together via the powerline connector on the rear of the Helixnet base station
You can connect a Helixnet basestation to a analog partyline system via one of the HelixNet modules.

 HelixNet Main Stations accept both two-wire and four-wire optional interface modules, offering  connectivity to virtually all common two-wire and four-wire audio systems

The HLI-2W2 Interface Module enables two-channels of Clear-Com or RTS intercom to transparently interface with digital HelixNet intercom channels.. Don't forget to properly null the channels attached to these modules and ensure there is a analogue partyline psu unit to power the HLI-2W2

The HLI-4W2 Interface Module provides an easy way to interface common four-wire audio paths such as matrix ports, cameras, sound desks and telephone hybrids with a HelixNet Main Station. Each of the two ports employ both transformer and electrical balancing to ensure incoming and outgoing audio paths are free of hum and noise.