multiple subscriptions on Pro Site?

I'd like to offer a couple "base" packages for hosting someone's site, and then a handful of add-on packages.

Is there a way to basically have multiple subscriptions? One would be the base package they want, and then another subscription for each of the add-ons. That gives them the flexibility to upgrade add-ons or switch them later, without disrupting their base subscription and without me having to have a million different subscription combinations of each base package with all the add-on variations possible.