Everything I know about adding smeltable stuff came from Meancloud's Mining:
Not sure if the mod's been updated for A21, but the basics should still apply. Basically, you'll want to first check out windows.xml in the XUi subfolder. This is where you add your new materials to the accepted forge materials, and add the additional slots to display them. Next, you'll need entries for your new ores and such in materials.xml. Then, in items.xml, you need both a "resource" and "unit_" for each of your ores. If I remember correctly, resource is the form used outside of the forge, unit_ is the stuff stored inside it. In recipes.xml, you'll need <wildcard_forge_category> recipes for each of your new ores, which I believe lets you smelt them into the forge, and a second set of recipes to run the smelted materials back out of the forge again. Plus, you'll put the recipes that actually uses the ores there, too. And finally, you add your actual sources of ore in blocks.xml.
Hope that gets you started. Like I said, see what's going on in MeanCloud's Mining and it should all fall into place.