Mod item result in vanilla minecraft item

Hi, I'm making a mod that for example...
You mine some stones and get iron pieces and smelt then in the furnace.

And to create a Iron bar you need a X quantity of iron pieces to make a bar.

I want to my modded iron piece results in a vanilla minecraft bar, someone can help me? XD

The first solution that I've found is...

When (my modded item created) = add 1 iron bar on inv (but if we hold SHIFT, the iron bar is added AND another one is added too (cause i'm holding' one 'bar' on my hand)

And I'm just wanting the result of the modded pieces is being an vanilla minecraft iron bar..