How to build k9mail with an file?

Hi !
I like k9mail and I would like to add it in the LineageOS builds I compile. However in order to do so I have to create an file in order to build the app and give it a package name I can later add in my product_packages (in order to add it in my builds).
I tested to reproduce some file of some other app without success :/.

Has anyone the knowledge of doing a such file ?