If you already have the cpanel backup tar.gz file it's pretty straightforward. You can do it ll via a nice gui web interface using VirtualMin's migrate virtual server function.
1) Install virtualmin (note you may need to install wget with "yum install wget")
2) Go to your new webmin install via your browser - [domain or ip]:10000
3) In the search bar at the bottom of the menu type: migrate virtual
4) Click on the migrate virtual server link and follow these instructions.http://www.virtualmin.com/documentation/id,virtualmin_for_cpanel_users/#copying_the_cpanel_backup_to_your_virtualmin_server
I ran into trouble when I tried to have it automatically work out the user/password so give it a user/password and it should work fine.