I believe your question is, would the 1.6 app work in a current 19.1?
That depends and is hard to say without looking in detail at the code of the app. Thought you can try it out.
The backup and restore does that for all apps (storing the data in the database), so that includes your extra app, given it uses just the database. Files in our files directory should be copied too, as I wrote before.
The 19.1 code PLUS the 17.1 egroupware-epl-compat package contains code to update the 1.6 database schema to 19.1. Just the 19.1 container is not enough, as it requires at least version 14.3 latest!
If you come to our hosting we would take care of importing your 1.6 data, but we do NOT support your own applications (for security reasons). We offer update packages, which includes installation of the latest EGroupware 19.1 plus import of your old data on a Linux root server:
Anyway, unless you try the update yourself and run into concrete problems, or contract us for hosting or updating your EGroupware system, I will close this thread now from my side. Hope you understand.