Add to the player's default rig?

I’m trying to create a running animation for my game that looks similar to this.
sonicrunning
My idea is to add a cylinder to the player, and push the legs inside the player so they aren’t visible, then show the cylinder and have it rotate with their pants texture on it. How would I go about adding onto the default rig?

I don’t want to create a new one, because I still want to have the player’s avatar as part of it.

So how would I go about doing this? There are also a few other ways I want to use this. I can’t find any ways to do this online, but if someone can find one please link me to it.

I’ve found out how to do this, any others browsing for solutions, here is a good tutorial for this.
https://devforum.roblox.com/t/how-to-animate-tool-parts-guns-knifes-etc/359484/42