Add-ons are assigned to Services, and Services are assigned to Staff Members. So any Staff Member that is assigned to a Service, will also be assigned to any Add-ons that are assigned to that Service.
As a workaround you could create two different Services. One with the Add-on(s) and one with out. Then assign those Services to your Staff Members based on whether or not they can provide the assigned Add-ons.
For example, Staff A can provide all Add-ons but Staff B cannot. You can create a Service titled Massage with Staff A, and a Service titled Massage with Staff B. The Add-ons would be assigned to "Massage with Staff A" and the Services would be assigned to the respective Staff Member.