In ASP.NET WebForms there can be one an only one form on the page and that form already exists so you cannot add additional form elements.
If you need a form to post to another site such as paypal, then you can use a button and set the PostBackUrl property on the button, then when the button is clicked javascript will change the action on the main form to your postback url before submitting the form.
Just remember I cautioned you NOT to modify the source code, you are going down the wrong path if you do that and I think you will later regret it when you cannot upgrade without losing your changes.
Hope it helps,
Joe