It seems to me that once you gather different dice from your different weapons you just roll them together as a single attack. But the questions comes from the different keywords that different weapons have.
Suppose a unit of Rebel Troopers is attacking. 4 of them are using impact grenades, they contribute 1 black dice each with impact 1. But one of the troopers fires his Z-6 contributing 6 white dice. The pool is now 4 black and 6 white with impact 4. Can the hits rolled on the white dice (contributed by the Z-6) be turned into crits by the impact (contributed by the impact grenades)?
I know it seems to go against logic, but I think they can. If this is not the case it is pretty clear that black dice can take advantage of the impact but white dice can't. But what if the weapons contributing the keywords use the same kind of dice as other weapons? Do you have to roll them separately?