Oh, I missed the question...!
There is a fairly easy way to statically add kcm files with the need to build and install a new apk after adding – not dynamically, though.
https://community.fxtec.com/topic/3848-pro1-x-us-and-us-intl-with-dead-keys-keyboard-layouts-finqwerty/?tab=comments#comment-65908
I cloned FinQwerty, I think I modified pro1x_qwerty_usa_1.kcm because it had a bug, then I created a new kcm file pro1x_qwerty_usaintl_1.kcm (to use myself), added entries for the latter in generate_layouts.py and in the finqwerty_layouts.xml and built a new apk (using IntelliJ IDEA