Can't find an answer, so I'm asking...

Fast Hands: After you activate attached character, you may resolve one of its character or upgrade dice.

So, if I roll two dice and one is a ranged roll, the other a modified range roll, does the modified range roll automatically add to the ranged roll when Fast Hands triggers?

No. Any card that allows you to resolve one dice lets you resolve exactly one dice, following all the normal rules (unless the card also allows you to break those rules, such as Awakenings). So you can't add modifier dice (because you would be resolving more than one dice), and you can't use it to resolve a modifier (because you've still got to follow the normal rules about resolving them along a non modified dice).

RESOLVING DICE THROUGH CARDS Page 9
Many cards allow a player to resolve one or more dice. When a player resolves a die through a card, they use the normal die effect based on the symbol, and follow any extra instructions.
A player must still pay any resource cost on that die.
A player cannot resolve a modifier by itself.
A player cannot use modifiers when resolving a die through a card effect, unless the card allows them to resolve multiple dice of the same symbol.