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.
As a workaround you could create two different services. One with add-ons and one with out. Then assign those services according to which staff member can provide the assigned add-ons.
For example, if 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.