how to create an add-on for readynas?

Because the ReadyNAS has such a small OS partition, you have to be careful installing ones that aren't specifically compiled for it.  Apps should go in /apps, which is on the data drive (with a link to it on the OS partition, if needed).  It's especially important if the app has a log that can get long or creates working files that might get lengthy and/or not get deleted properly.  With some, that's as easy as making an appropriate directory in /apps and putting links in the OS partition.  But if an app uses a standard directory that you can't relocate because the OS itself uses it, then you'll have to re-compile, changing that directory to something that can be in /apps.  Misbehaving apps are the prime cause for users' OS partitions filling up, which can result in some significant issues if not caught and corrected fast enough.

But even if an already compiled app isn't a problem, there are no simple instructions available on how to convert it to a ReadyNAS-specific app that puts the files and directories in /apps and necessary links in the OS partition.  I created new sources.list and apt.conf files to get around the issue that Debian 8 has been archived, but I can't figure out how to make a simple app that installs them in /etc/apt.  And those aren't even executable.  And I do program tools for my own use in Windows, so it's not like I don't have any background for it, I just don't have Linux experience.