I'm not familiar with cpanel (or da for that matter), but it shounds like what you want to do is create a apache virtualhost on a different port, and have the document root for that contain the webmail files.
I'd guess DA will overwrite any manual changes to the httpd.conf file you make, but I did notice they have a form where you can edit hosts.
My suggestion. Create a site in DA.
in the admin panel, click administrator serttings, at the bottom click Customize Httpd Configurations. Click on the site you just created. Where it says:
<VirtualHost YOURIP:80>
change the 80 to whatever port you want it on.
Click save.
Again i'm not familiar with DA, but you could change the document root in that file (DocumentRoot /location) to the location of the webmail pages... OR... install your own webmail software into the docroot for that site.
I havent tested this but it should work.