michalk As I'm not Sketch team, I can only provide guesses for you ?
I think better support for adding a remote library from RSS is coming in a future Sketch release, so hang tight!
1) There are methods on the MSAssetLibraryController
such as remoteLibraryWithAppcast:
and - (void)addRemoteLibraryFromAppcastURL:(id)arg1 withCompletionHandler:(CDUnknownBlockType)arg2;
which look like they might do what you want, but I haven't tested.
2) As far as I know, there is no support for library URLs in the plugin manifest.json