You could definitely explode a d20 into a d8:
d20!{20=[d8]}but the d8 will be added to the d20 result because it is part of the same dice pool. To sum the damage pool with the exploded d8, you would need to look at the dice and do the sum yourself.
So whilst it is possible to roll both attack and damage at the same time, I would recommend rolling the attack first and then rolling damage afterward if the attack hits (this saves good damage rolls from being wasted too ;p)