It was a very recent feature (my version is 2.3.7). Is your Beersmith software up to date?
IBUs for a "steep/whirlpool" hop addition should be the same as a zero minute boil addition (as long as the "steep time" is the same as the configured "whirlpool time above 85C").
There is also "steep/whirlpool util factor" in Options/Bitterness and "Steeping Efficiency" in Options/Advanced. 15% in mine which I think is the default? These settings have an effect on non-zero minute hop additions but then the calculations are a bit more involved!
(EDIT: Apologies, the "whirlpool time above 85C" is also a setting in the Equipment Profile and probably overrides the same setting in global Options. And I guess the partnering setting "estimate boil hop util in whirlpool" in the Equipment Profile just turns all these attempts to judge the IBUs off.)