First you will need to install the new cms (in a folder). You will need a designer to adapt the template if you want to keep it. Also there are extensions to import content from one cms to another.
JConverter is a good tool to import your content - http://www.sopantech.com/services/free-extensions.html As VideoWhisper says, the theme will need to be rewritten to enable usage with Joomla.
Omar, yes, as CL mentioned above, JConverter works well to convert from WordPress to Joomla. I had to do this for a client one time, which I did not want to do because I far prefer WordPress
Hi, I think you may be interested by a joomla component that allows you to use Wordpress inside Joomla ! Make a google search like that : "wordpress for joomla corephp" Hope it will help