If you need additional Zimbra mailbox licenses then you just submit an order for the number of additional licenses required in units of 25 mailboxes. The minimum order quantity is 25 mailboxes.
Agilemail will process the order and then you will be issued with an updated Zimbra license. This new license includes existing plus the additional licenses.
Fees are not prorated, instead any difference in fees are discounted against the next license renewal.