I have a ZeXtras Suite license but I need to add more mailboxes, can I simply buy a new license and upload it to my server?

No. ZeXtras Suite only allows 1 license file to be active at a time, so uploading a new license will overwrite the old one. Should you need to add more time/mailboxes/modules to your current license, you can use the "Upgrade License" feature found in your ZeXtras Store account.