Have asked itch.io about the ways to do it. Will let you know again once I've received an answer.
Okay, thank you!
Itch.io has responded, and "topping up" previous purchase isn't something that is currently supported. Currently, the only way to do it would be to ask for a refund, and then re-purchase with a higher value. If you want to do this, send me an e-mail to email@example.com with your itch.io account e-mail address (I'll relay to itch.io regarding the refund).