Make Install-able Python third-party Submodule

I have a pip package that I want third party developers to add to it.

Meaning: the 'root' pip package has the basic interfaces and abstract classes and it can detect sub-classes. Now I want a third-party developer to write another module that inherits classes from the root package (the 3rd-party submodule has the root package as a dependency) and consequently the root package can detect subclasses from the 3rd party package.

So by the end when in a script typing something like that: import mypackage.* the root package and all submodules are imported

Any recommendation for a good practice to do that?

Thanks