There is no way to purchase an
Office 365 tab in
Request #APSA-18279: "It is not posible to buy add-on to existing license in Office 365 tab" has been addressed to Odin Development team in order to implement such functionality.
Contact email@example.com or TAM to check the status of the request.
Workaround: use the following CCPv2 screen to adjust
CCPv2 > Account (tab) > Subscriptions > [a subscription] > Change Resource Limit.