all the info you could ever want about modding any Big Muff, look
here. to add a mid pot
Replace C1 with a 22n cap, and replace R2 with a 22k to 25k potmeter (wired as a variable resistor) with a 1k resistor wired in series. so take a look at the schematic to find R2 and C1, match it to the layout and you're good to go. it's something i'm looking at doing with a future muff, along with a bunch of other mods found on that page.