Plug-ins are not supported at this time and may never be.
Plug-ins were deprecated a few years ago.
DSM 7 is a complete re-write.
We were not able to make Plug-ins visible to the user through a GUI.
I am looking at trying to implement something, under control of the installer, but am dependent on Synology for API information. They are not likely to provide that information until after DSM 7 is in final Release.
