Ok, this works for me (excuse the odd naming) :
Expose a project as “subsub” that contains a form “Form1”.
In your main project, add the dependency “subsub” and the following code :
import subsub
from subsub import Form1
...
new_form = Form1.Form1()
self.add_component(new_form)
You may be able to simplify that if you’re better at Python than me, but it seems to work.