The focus here is on the base oil composition rather than the additive package. Certain OEM approvals require the use of Group III base oils, whereas AMSOIL, for example, prefers to utilize PAO or GTL. Regarding the modification of additive packages, suppliers provide blending guides that detail how these packages can be top-treated. It's analogous to assembling Lego, but with intricate chemical compounds. This process should not be a cause for concern.
I've seen this from Infineum for example, where they sell Trimer Moly separately for certain additive packages, and leave it up to the blender if they choose to add it. In this case, it's for additional fuel efficiency in certain lower viscosity motor oils.