RFE: Multiple Upstream Tarballs support

I'm dealing with a package where a few files are added that are not shipped by upstream. Building a source package, the same files need to be present e.g. in distro/pkg/deb and distro/pkg/rpm, making it quite error prone to maintain (imagine you add a new one, you might forget to add a copy in the other location/s).

Things like symlinking don't seem to work (not surprised), is there a way to share files between packaging styles from a common location?