Promash doesn't support late addition of extracts (which affects hop extraction). Either that, or I can't find out how to do it.
I sometimes use beercalculus.hopville.com to create two recipes, one without the late addition, from which I take the IBUs, and one with the late addition, from which I take the OG.
Is there any software out there that will allow me to note when fermentables go into the boil?
UPDATE: hopville.com has supported late additions since shortly after I asked this question.