Apologies if this has been asked; I searched and didn't see it.

If I have Backdraft w/ AC shooting out of aux arc can I roll, cancel & add 2 hits THEN use Backdraft's ability to add the crit?

No.

Adding results is one of the three types of dice modification. AC blocks further dice modification after use.

For completeness, the other two are:

Changing results (e.g. Autothrusters)

Rerolling dice.