Add "Skype" field to Membership registration and profile

As part of my Membership registration process I'd like users to provide their Skype name, as well as have this field exposed in the Membership profile page. How can I do this?

Patrick suggested the Profile Builder plugin, but it looks like I'd need the $50 to add custom fields, and it still wouldn't be exposed in the Membership registration process.

I do have a simple plugin installed that can change the "AIM" field to read "Skype" which might enable me to use this existing field, but again, I need a way to expose it during Membership registration.

Thanks in advance.