Yes, I believe that you can make one up.
You posted this in the wrong section though. If you have any further questions like this you should refer to the "Runescape General" section.
You can make it up, but really I would make a real one you have access to. This way it's much easier to recover it if you forget it or someone steals it.
No, i just tried to make a new account and you must activate the account with the E-Mail.
So you have to make a real E-Mail, before you could just make one up.
You can use a fake email as the login email but then you'll need to "resend" your activation email to a different email. This email has to be a real one.