There are certain combat cards that let you add units to an existing combat. Space Marines have Drop Pods and Chaos adds cultists left and right.

When you add a unit in this way, do you get to roll their combat dice and add it to the fight?

No. Combat dice are only rolled at the beginning of combat, or when cards tell you to. The point of generating new units is more for variety (for additional combat card triggers), resilience (more units about means that your army can last longer and has a higher morale value), and strategy (generating units for free doesn't hurt).

Thanks for the heads up. For Drop Pods, you don't get them for free. They have to come off the boards from another system.