I was working on setting up a role migration from Drupal 6 to Drupal 7. After some time, I noticed that the role id's were not being preserved, which was going to cause a problem for me when I went to take the migration to the live server. I initially tried using the addFieldMapping function to link the source rid to the destination rid (similar to preserving the nid for nodes or the uid for users). However, this failed to save any of the roles during migration.