It appears the git-plugin is already capable of merging another branch into the current one. It is probably not designed for this purpose, but after digging into the source code i found it is possible.
In the merge before build step, the Branch to merge to can be specified. Here the branch to be integrated can be specified. In the git publisher the Merge Results checkbox indicates if the merge is to be pushed. It will than push to the branch which was merged, not to the current branch being build. Instead of checking the checkbox, Branches to push to remote repositories can be specified. If the current branch being build is specified here, the merge is pushed there.
This usage may not be intuitive, but it is sufficient.
Thanks,