The rules are very simple: you need a LH marketed flight number. The rail ticket is issued separately and doesn't need to be added as a segment in the booking. LH just needs a LH flight number so it appears for them to crossreference it in their native GDS.

Other airlines also participate in the DB R&F programme. Most TAs can add a R&F to any flight booking in Germany. They will charge 5-7€ for the work though.