Ok, so what I did to make a custom package group in Katello local repositories:
Download the repository to my local machine, make the changes I want and then sync it back to the Foreman server.
Don’t know if this is the correct way, but it’s the only way I can think of right now. The thing is that I still can’t find where Katello stores all the packages, after sync. It should be somewhere inside /var/lib/pulp I think, but I can’t find any repo structure over there, only a lot of used space full with temp files and stuff like this.