Yes — depending on your point of sale, you can build a challenge or campaign around a specific modifier rather than a full menu item. One customer example from the webinar built a challenge entirely around adding sprinkles to an order a set number of times.
To do it, don't search the regular menu item picker when building the campaign. Instead, type the modifier's name directly (for example, "bacon") in the item search, and AppFront will surface the matching modifiers so you can add them to the campaign the same way you'd add a menu item.
This opens up much more specific and often more fun campaigns and challenges built around add-ons, rather than being limited to full menu items only.