Client-side encryption of messages across apps
No. Apps on Android can't interfere with each other's operations to the extent you describe. I can think of two options that come close to what you describe:


Use a standard PGP app and copy-and-paste ciphertext between that and the chat app.
If the chat app you're using uses direct client-to-client transmissions, instead of every message going via a server, then you could set up a VPN and ask your partner to join it before starting to chat with you.


To be honest, neither of these options, nor the ideal circumstance you describe, sound like an easier sell than simply asking your friends to use a secure chat system. I suggest to you that Jabber offers this functionality while allowing a choice of clients, some of which can also use different chat systems, so it might be the easiest transition path.