How to create frames for a new critical for an existing animation?

So, say I wanted to port the animation of Eliwood summoning Roy as his Stand into FE7, but wanted to keep Eliwood’s normal attacking animations. Would I be able to ONLY create critical frames or would I have to essentially remake Eliwood’s animations?

Use the newest version of FEditor to dump the anim with frames included, then change it as needed and build it again.