Is it possible to link a WHMCS Addon or Configurable Options to a SolidCP Addon?

Say I have an Exchange Hosting Plan with 1 mailbox and in WHMCS I want the customer to be able to add on additional mailboxes how do I map this into a SolidCP Addon?

So far I cannot see any way to specify the PlanID in WHMCS Addon or Configurable Options. I have only been able to do it manually.