I used the following command to add an existing project as submodule to my project.
git submodule add https://github.com/...
path = name
url = https://github.com/name
I believe the solution is to add your submodule to your project, and than add a symbolic link into the actual package next to the submodule (or where ever it makes sense to put it)
So you'll have
project - submodule - package - setup.py - my_awesome_script.py - package (symbolic link to submodule/package)
then you can just import it through the symbolic link.
import package package.work_your_magic()