I don't think they are reloadable (the regular ones). They come in standard amounts (not set), so I'm guessing no.
I have done the online gift cards though. Good presents from people
You just need the Sony software to buy them and an email address to the person you are sending them to. So if you needed to, you could send them to yourself. Important note: I've had them take up to a day and a half to process and email the gift certificates.