There's no feature to assign dedicated ip to an existing site in the control panel, however, you could assign it manually. You will have to do the following:
1. Modify the site bindings manually in IIS.
2. EDit DNS zones in Websitepanel
3. Update WebSitePanel database manually. You can run the following sql script:
where PropertyName ='SiteId' and PropertyValue ='SITE_NAME')
select@addressid =(select AddressID
where ExternalIP ='IP_ADDRESS')
set ItemID =@itemid, IsPrimary =0
where AddressID =@addressid
set PropertyValue =@addressid
where PropertyName ='SiteIPAddressId' and ItemID =@itemid
Note: Replace SITE_NAME with the site name of the site you need to specify dedicated ip, replace IP_ADDRESS with the dedicated ip which you will assign to the site.