Add user groups when replacing data

[theskyfloor]

Is it possible to add user groups to a user that already exists upon import - right now it just replaces their user groups assignments complete but I want it to be able to add up. So import once so that person 1 gets added to group 1, import another set of users and if person 1 is in second set he is then added to group 2 and is still part of group 1.

Is this possible? or maybe its a FR.