It can depend on how much you want the damage separated for damage resistance but if that is rare, you can just add it to the crit field. For example, here is the crit with multiple damages in the roll itself

You will have to mouse over the crit damage but it can work if the resistance issue is not common. So in the crit field, instead of just 1d6, the field is "1d6 [cold] + 3d4 [lightning]" (note you can also do [[1d6]] [cold] + [[3d4]] [lightning] to add the dice up nicer if moused over, will just remove the green/red outline of the damage). A similar thing can be done with the plus 10 you mention. Just put 1d8 + 10 in the crit field for the normal damage
EDIT
If the damage is a whole third damage type however, setting up a companion attack could work as it needs both the normal hit and crit damage. Doing this I would make a new attack, only fill in the two damage fields (first for normal, second for crit) and auto link it to the first using %{NAME|MACRO} syntax in the freeform field. More on that can be seen in the documentation
https://docs.google.com/document/d/1yPcIZ_bIc3JlnW...So crit only can be handled in the crit field, a full third damage type needs an extra setup as far as I know, all 5e sheets only support 2 damage types on an attack.