You can use it inside the same repo. Provide a whole path from the highest repo level in any notebook inside the repo. As you mentioned, if the file is in another repo, you need to use sys.path.append. To make it permanent, you can try to edit global init scripts.

from directory.sub_directory.my_file import MyClass
 
"""
Repo
-------\directory
------------------\sub_directory
-------------------------------------\my_file 
"""