Hey Devil Monster,
If what you want to do is change the animation for the Rockclimb Hm, this should help. Otherwise be sure to get back at me on this topic. :)
Make sure your Rockclimb event looks something like this:
https://imgur.com/a/jJJGEF8
If you click the edit option on the "Set Move Route: Player" part, a window will pop up in which you can adjust the animation(Change Graphic...) and set the movement Speed(Change Speed....). You'll wnat to revert those at the end of your animation though.
If you want this to be used as an item instead of an Hm you should replace the line "Kernel.pbCheckMove(PBMoves::ROCKCLIMB) != nil" with something like "$PokemonBag.pbHasItem?(:INTERNALNAMEOFYOURITEM)".
If this helped you I'm glad, if I didn't enough be sure to ask for clarification. :)