It took me a while to understand the question, but I guess you're asking how to setup your own asset library and connect it to Godot.
To add a new repository, for the time being, you have to edit this file: https://github.com/godotengine/godot/blob/master/tools/editor/asset_library_editor_plugin.cpp#L1323-L1326
In the future it should be possible to add repos without having to compile the engine.
As to setting up your own instance, you need to expose an API that can talk to Godot. There is no official documentation yet as to how to do it, but you can check the code of the official asset library: https://github.com/godotengine/asset-library