I do not understand your problem. Do you want to switch to a paid hosting, or want to use the domain that you bought with your free hosting 000webhost?
If you want to use your new domain with 000webhost your free site, you need to configure your new domain to use the 000webhost nameservers(ns01.000webhost.com and ns02.000webhost.com).
Then you must use the "Parked Domain" you can find on 000webhost control panel.
If you want a premium server, you should contact http://www.hosting24.com
which is the company that keeps paying hosting 000webhost. Do not forget to tell them that you already own a free account on 000webhost.