Hello Susanna,
While it is possible to export the user/group information on the version you are running with the csutil -d option, the problem is extracting this information and import it back to the IAS Server database is going to be very difficult if not impossiuble . If you are comfortable with script and can understand the extracted information from the csutil -d text file, then you may be able to do it, but understand that this is not supported by Cisco.
rddms is only used for importing the data to ACS from another RDBMS data source.
Thanks,
Mynul