You can create a miscellaneous item for your whirlpool only step.
OP's suggestion comes from another thread where the misc item was suggested and explained. I think OP is looking for something more purpose built that's independent of hop additions. As it is, WP hop additions are counted from the beginning of steep, rather than being able to add them as part of a countdown to the end of WP (like boil timing). BeerSmith doesn't allow more timed process steps after the beginning of steep, so a brewer has to custom create a bunch of timed steps calculated out from the start of WP.
As BS evolves, I hope to see WP become a countdown addition, like the boil. As an aside, I'd like to be able to add many more fermentation steps, similar to how I can with a mash.