OP, I understand what you look for, but that’s not easy task. From my limited knowledge of apps development, achieving what you requested would likely be:
Identify and remove all relevent code to the backend. Easier if it’s modular, very hard if they’re litrered everywhere.
Chose a XMPP client library that have relavent extension support that can translate to features in XMPP.
Write an adaptor (if modular) to match the methods signature and translates calls to the client library. Or reimplement all the code you removed (if littered everywhere) with the client library.
This is akin to swaping to a new engine for a car, with incompatible mounts. Diffcult to execute, and (I believe) low interest. You can try if you got the skills. I don’t and even I have, I will just use SimpleX which fits my needs.