There is already an avenue for people to donate to you if they like your hubs. It's called "adsense". Anyone liking your hubs can click on as many ads as they like. This is what pays you; so, if you have not signed up for "adsense", I suggest that you do.
Putting a paypal button on your hubpages would only put people off donating to you. I don't think it's possible, however.