Patrick it is do-able.. But to do it good will cost you more money then you had boughed a Talkback converter..
Mixer to camera is easy! Get an audio to sdi embedder and set it up to route 2 audio channels to sdi channel 15 and 16. Put this SDI line into your RCP and now you have audio to the cameras.
You will need to add a box that can interface between 2wire RTS and 4 wire audio line level audio. Prefferable with audio levels settings.
Then for every camera you want to hear at the mixer side.. You need another SDI audio de-embedder. Put between the camera feed justbefore it enters the mixer. Then set it up for audio channels 15-16. Feed this audio into an audio mixer to add them all together to 1 feed. And this feed goes into the that RTS 2W to 4Wire interface to go back onto the Partyline.
All in all this will cost you about 1 Embedder, 4 de-embedders, audio mixer, and 1 to 2 days to get it all set up and working.
Might be much cheaper to pick up an older Talkback converter on ebay for $500
In both situations you will also need a 2W to 4W interface to run to your RTS system..
You need something like a
RTS SSA324 or
Clearcom IF4W4