This guide is pretty badly written, but it gets the idea across. One, and only issue I found was, the dynamic shadows of the preexisting mane will stay, as will the bones, but that can be worked around with, at least in stills, I don't animate, I dunno if it'll be a nightmare to animate these.
Getting started, with a mane. Tails are pretty much the same, except a few changes noted below:
Open up your map, and all that usual stuff.
Spawn the pone, spawn the mane you wish to use.
Add material overlay to your Pony, then open it in the element viewer under model.
Go to the hair(Sometimes it's hairfront and hairback, or could be something els