JS or PHP PayPal Extension to existing Form

Attached you'll find the HTML ouput of an existing form of the wordpress registration process. I need a small PHP or Javascript PayPal Processor to extend this form : After the user fills in the fields and press the submit button he will be redirected to paypal to pay something. If he payed, the default action will be processed. If not he will be redirected to an error screen. What's the Problem : I can't change the form. !!! I only could extend (add some lines) into the header or i could add some lines between the formfields and the submit button. Again. I can't change the given form.