As far as I know, it isn't really possible to load certain resources, like 3D models, outside of the project resources in Godot right now. I made a GitHub issue on this, but as far as I know, there is no way to import the source files for certain resources in Godot.
If you are okay with converting the files to a format Godot can use, then there are some options. One option is to use .pck files that contain all of the data you want to import. This is probably the best way, since it is officially supported in Godot and allows for importing any/all resources. 3D models, textures, scripts, and more should be importable with .pck files.
I have not used it myself, but there is a tutorial on the Godot documentation that explains a bit about how .pck files work.
Another option is to export the files using Godot's file class, and then import that. I made a plugin for this, but like .pck files, it requires an addition step and the source files (like .obj) cannot be directly imported, but rather they have to be converted first.
Finally, another option is writing an importer in GDScript or C#. Here on the forums, a user wrote their own OBJ parser in Godot (GDScript I believe) so they could import .obj files outside of the project in Godot. Here is a link to that topic if you want to take a look.
Hopefully this helps a bit.
Also, welcome to the forums!
