Right now it is not possible to easily edit users' roster while the user is not connected. The only way to do so is by manually loading the roster (for RosterFlat), parsing it, editing, serializing, and sending notifications. We are doing that in UserRosterManagement.groovy. A very similar thing needs to be done in MIXProcessor to actually edit a users' roster to add/remove a MIX channel entry while the user is not connected.

It would be good to have a single "working" implementation in one place instead of having the same code in multiple places and be able to support any roster implementation (not only RosterFlat, which is now the only supported roster implementation for such editing).