The problem is that paypal buy now buttons submit directly to paypal and ignore any form fields that don't relate to what is directly supported in their api. Which means your extra form elements will just be ignored and you won't be able to do anything with them anyway.
So adding the buy now button to an existing form won't really do much good if your goal is to somehow capture the form element values when the buy now button is pressed.
You would probably have to use something like eCart and express checkout to be able to capture form element values and send someone to paypal at the same time. A buy now button just doesn't support that type of combined form.