I would like to add my support for this feature, as I frequently use atypical ingredients for priming, such as brown sugar, or liqueurs. Even specifying the type of dry malt extract is important of keeping track of recipes.
I'm only testing BeerSmith right now, and I find it to be a very useful program, but I find this a big limitation given my tendency to experiment. I can add ingredients for anything else, it would be nice to add my own priming sugars as well, not to mention an expanded database of commonly used ingredients.