You should be able to edit the WHM > Edit DNS Zone and change the CNAME or A record to an A record pointing externally. I've edited DNS zones on my cPanel machines before in WHM to change the location of the www.domain.com versus the domain.com entries.
To show this is possible, I changed my www.tristanwallace.com domain to point to another account and didn't revise tristanwallace.com location. This was simply done by editing WHM > DNS Functions > Edit DNS Zone, selecting my domain, then revising the following:
Code:
www 14400 IN CNAME tristanwallace.com.
To the following:
Code:
www 14400 IN A 208.74.124.112
At this point, the two locations show different content. I'm uncertain how precisely you are editing these zones and what you are putting for the zone details. If you are hosting the zones externally, you should still be able to do it like I've noted above on the external machine to have www having different records. I've done this at my domain registrar before as well.