You can't add it to the paypal form I don't think (you would have to ask paypal to be sure, but I haven't seen that option). But you could add a checkbox on your checkout page with that option on your site. I think they charge $0.30 plus 2.9%... so you could add a charge rule to charge that amount if they opt into it... Just save a session variable based on the checkbox value and use that as a condition in the charge rule.
Maybe I don't understand the question... This is a donate button to begin with? You see the option when they checkout, but you want it on your site too? Or are you asking to add this option outside of the donation context?