It would be great to have an RDA Helper like MarcEdit has (which is not open source - http://marcedit.reeset.net/features) built in to Koha so that you can easily add RDA fields to your MARC record with the click of a button or via a batch biblio modification tool.
From the MARCEdit homepage: RDA Helper: The RDA Helper is both a stand alone and integrated tool that allows users to take a set of MARC data and automatically generate RDA specific data. The RDA data is derived from the source recordset using available control data, variable field data, and RDA specific assumptions.