Very confused about how the APIs and merchants work

I'm very confused about how my API works with outside merchants. I've written a plugin for a shopping cart that other merchants should be able to use. My intent is to sell the plugin for a one-time fee. How do I give the merchants access to use the plugin? I was lead to believe that they would contact Payeezy and would be added to my Merchants tab here on the site, then I would send them their tokens. Now I'm finding out that all anyone seems to be able to do is create NEW merchant accounts. My first customer has 17 payeezy merchant accounts from an ISO. He doesn't want to create new accounts. How do I add these to my list so he can use my plugin?