It seems that impex nulled all birthdays to a date and users are not able to change it. Now this board that we are talking about here has over 6000 members and as you can imagine it's a lot of work to change them manually one by one.
This add-on should have these features:
-Select usergroups which have the permission to change their birthday
-Users which have a certain day can change their birtday example: 01.01.2011
-Limit the time which selected users or usergroups are able to change their birthday.