How Park Dedicated Domain on WordPress Multisite ?

Hi ,
We buy WHMCS MU Provising and install it on WP and On WHMCS and Work Correctly , But We have a Problem that We dont know how we Can add a Domain in Weorpress ?
Now When Order Wordpress Plan in my WHMCS, when client order a Dedicated Domain For Example http://www.sis.com or when select "set DNS domain to my Server NS and enter Domain (www.sis.com) or when select use From Free Subdomain and use from this subdomain for example : sis.mysite.com , in three states ,WHMCS not use From Dedicated Domain that entered by Client and generate Subdomain in this three states for example : sis.mysite.com
Now We want ask for you Can We Config and set dedicated Domain For Client's Wordpress Sites ?
and We how can set DNS for Send to Clients that Dedicated Domains DNS Set to my DNS's and Domain Connect to Clients Wordpress ?
thanks