This program allows you to append frames to another set of frames. It does this by renaming files "up" so their numbers are listed ahead of the first set of frames. What makes this different from "Renumber frames" is the ability to change the text before the frame numbers.
This isn't technically a plugin, but I feel it should be.
Let's say you have MyMovie_000001.xcf through MyMovie_000500.xcf that you need to append to an identical set of files.
Move the executable into the directory where you want to append *from*.