Should I add an escape register for added ventilation in A/C duct to reduce condensation?
The hot attic has more latent heat (water vapor) due to both it's higher temperature and being vented to the outside. As that air mixes with the cooler interior air, relative humidity rises. When the more humid air hits the cold uninsulated duct, it condenses.

The first step is to seal the opening between the attic and the interior space to reduce convection and water vapor transfer between the attic and conditioned space. Insulating the duct would be the second step.