This was an idea I had a while ago. Consider this an optional rule; I'd love to know how it works out in play.
You can activate a Theme that someone else has by spending a Twist. The stuff in the Theme still happens to that character and not to yours. If you need a Complication to do it, the Complication happens to your character and is based on your own Import level (though the other character's player can decide to take the hit if he/she prefers). Think of this as making the themes say, "This kind of thing happens to my character even when he doesn't expect it, or didn't ask for it."
I was reminded of this by a discussion at RPG.net about giving starships more character - by giving them Themes that people could activate by spending their character's own Twists.