JEI says create copper should work for tinkers recipes but it doesn't. I can't make bronze with the melter even though I have glass and copper. For some reason there is no way to add copper when there's glass in it and vice versa, even though it technically should have enough space.

Edit: It turns out you can make it in the big smelter but not the melter, nor using an ingot gauge.

The basic melter does not support alloying, full stop. This is a core mechanic of Tinker's Construct and cannot be changed by modpacks.