Are You going to bust clay pigeons just for fun, or are You reloading for a serious hunting application?
I confess to avoiding Pyrodex at all costs, but have reloaded shotshells with real blackpowder for years.
Get the fastest burning granulation of Pyrodex available. The blackpowder equivalent to 3F/3P/PPP.
Dismantle the steelshot cartridges, discard the smokeless powder, and cut off the bottom "shock-absorber"-part of the shot-cup incl. the cup-part facing the powdercharge.
Only keep the cup-part that encapsules the steel shot.
You will need the space gained to get enough Pyrodex in the case.
Especially since You're going to use steel-shot, which is lighter than lead and requires more velocity in order to "kill" as efficiently.
Buy some cardboard 12ga. "Over-powder cards". Can be had from Dixie Gun Works, Track of the Wolf, etc.
These will keep the hot-burning Pyrodex from melting the plastic shot-cup in Your barrel.
https://www.trackofthewolf.com/List/Item.aspx/171/1As to the amount of Pyrodex....I have no idea....I have only heard that Pyrodex is lighter than real blackpowder. You should load the same VOLUME of Pyrodex as You would real blackpowder, NOT the same weight.
The old rule-of-thumb is 1/3 of the cartridge volume should be taken up by blackpowder, 1/3 by the cushion wad(s), and the final 1/3 by the shot-load.
You might have to increase the powdercharge up from 1/3 and reduce the cushion-wad part accordingly.
I can state that 83 grains of 3F granulation blackpowder behind 28 grams (1 oz.) of lead-shot is the standard load in British "Gamebore" 12-67 cartridges.
I have used up to 95 grains of blackpowder behind 36 grams of lead-shot in all-brass shells. It is a "frisky" load.
You will have to do a bit of experimenting, unless someone else chimes in with specific load data for Pyrodex in 12 ga. shotgun shells.
The basic procedure is: "a suitable amount" of powder; one or more "over-powder cards" to take up the space between powder and shot-load; the plastic shot-cup with steel-shot; finally a star-crimp or roll-crimp. (in the case of a roll-crimp You will need a thin cardboard "over-shot card". These can be bought from the same suppliers as the "over-powder cards", or stamped out from milk-cartons.)
Important notes: You do not want any air-pockets inside the cartridge case, so You'll need the whole loading column to be lightly compressed.
This means that You can NOT use .50 cal. Pyrodex Pellets in a 12ga. cartridge.
I guess You can use 12 ga. Pyrodex Pellets made for muzzleloader shotguns if they can be had, but Pyrodex in loose granules gives You more flexibility when loading a shotgun shell.