Included Users per Plan

I know that spark has the option of max users on a plan, but is there a way to have included users per plan? Ideally, I would then like to create another option for them to add additional users to that plan. So that part would be like multiple plans, or an add-on to the primary plan.

So essentially, the primary user can invite/add users up to the included limit, then after that they’d pay an additional seat subscription against the subscribed plan.

If this can be achieved, what ramifications would there be if the subscription plan was changed to include a greater number of included users?