In order to add registered domain in wordpress blog first one need to change the nameservers of the domain as mentioned below:
Once the nameservers changed one need to add the domain via below mentioned steps:
1) Login into wordpress admin control panel.
2) Go to settings.
3) Click on domains.
4) Provide the domain name.
5) Click on add domain to blog.
To add this domain procedure is called domain mapping with wordpress blog.