As a matter of fact, Kontalk is 100% XMPP compatible + a few additions such encryption and a few other things. But for the rest, you can use a normal XMPP client to connect to it.
The problem is that Kontalk uses client certificate authentication. It’s not a custom thing, it’s very much standard, yet there is lack of support from clients. That I know of, only one desktop client supports it: Gajim. I don’t know about iOS clients, but I can’t find one.
I recently wrote a Pidgin plugin that also handles the client certificate authentication part (and also encryption and key retrieval), so you might say Pidgin is compatible.
Even bigger than authentication, another problem is registration. Given an existing account, and if ChatSecure indeed supports client certificate authentication, you just have to throw it the kontalk-login.p12 file from the personal key pack exported from the Android app after having registered.
On a different note, I’m actually thinking of building a web-based registration module that just generates a personal key to be imported in any client…