With JRFTL you can convert Rotten Flesh to Leather. By Default, leather is created by smelting rotten flesh. However, when the HardMode is enabled in the config file, first a prepared flesh has to be crafted from 4 rotten flesh and then smelted in a furnace.
With version 1.2 and newer you can also smelt leather in a smoker (twice as fast).
You are free to use it in any Modpack, just link this site. And it would be nice to send me, or comment, a Link to the Modpack, if it isn't on Curse.