Is this possible?

If I iframe a website that has a signup form, and under the iframe (on my webpage), could I put a button/link that would "submit" the form that is in the iframe?

Sorry if this doesn't make much sense... Basically trying to do what people do for CPA Co-Reg offers.

yes, you can use JavaScript to create the form submission postback, and it is capable of interacting with things inside iframes.