Can I use my current Pioneer DJ account, which is already registered?

I registered my account with rekordbox

Yes, just use the email and password you signed up with.

I registered with KUVO

Yes, just use the email and password you signed up with. If you registered via a social network, you can log in with that social network. 

I registered on the Pioneer DJ Forum

You can use the same email you signed up with, but we'll ask you to update your password after you’ve logged in. If you previously signed into the Forum using a social network, you'll need to create a new account.