most basic way is to keyframe the crop
if that doesn’t give you enough control, the next level up is to keyframe a garbage matte, 4 point, or 8 point
both of these approaches have issues due to FCP not having much control over the feathering: it’s universal, same feather on al sides including the edges of teh frame where no stitching is required.
one way is to layer a feathered version on top of a non-feathered version with a slightly offset animation,
if you’re very lucky a wipe transition might work, but you;d have the least amount of control over that.
or use motion
nick