Hello Christof!
There is a Development section on the wiki that should help:
https://wiki.ipfire.org/devel
and it includes a section on submitting patches:
https://wiki.ipfire.org/devel/submit-patches
For me it was over my head and I was unable to figure it all out. So if that happens, you may want to head to IPFire Bugzilla and submit your Feature Request there.