You can add widgets using jass or add/remove dummy spells with overhead icon attachments using gui, i advise you the second option. Create a dummy spell based on some aura, set it's targets to self, area to 10 or so, attachment point to overhead and model to whatever you like. Then, when a quest is to be taken, set that spell on the unit and it's model will appear over the unit's head. You can put basically any model to such use, some good ones can be found on the hive, especially under 'buffs' section.
