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Has anyone done this? Did you fill out the (free) listing change form?
Slip em' another submission fee with the reason for the request to be able to take advantage of the sponsored listings?
Just contact a sales rep - can they bring about a cat change to make the sponsored sites an option?
Was it a free submission, or did you pay for it? If you payed, you could email the editor that accepted your site, and plead your case with them. I've had success using this technique.
If it was a free listing, the best way would be to pay (if you can), and add a paragraph stating what you're trying to do somewhere in the submission form. If you can't pay, then you may want to use the form and follow up by calling them.
Good luck!