aww ye, gather 'round mah jelsa shippers
Anyway, like the others said, it would be better if the Frozen storyline stayed the same; however, throwing Jack in there somehow might be possible. I've seen fanart on tumblr, which is what I'm basing this off of: Jack helps Elsa realize her powers do more good than the evil she thinks they do. That's how she figures out how to let it go 'n' love her ice powers. After Anna leaves when she's shot in the heart with Elsa's magic, it could be Jack that stops her from destroying herself, because Elsa knows she really hurt her sister.