Exploding for D&D makes it a bit wonky because it adds them together... So if you were doing 1d20e20.. Then rolled a 20, then a 13 on the explosion, it'd tell you the total is 33.. I'm sure you could just do the math yourself, but still wonky.. Adding the critical confirmation roll after you know you've rolled a threat seems like the best solution.. Or just roll the d20 to confirm when you do everything else and just ignore it if the original attack roll doesn't come up a threat.