Linux DHCP Server

I already have an existing DHCP server can I use that instead of the one provided with FOG?

Yes, do the following on the FOG Server:

 service dhcpd stop
 chkconfig --level 345 dhcpd off

Existing DHCP server

You must add the following lines to your existing DHCP server:

 if substring (option vendor-class-identifier, 0, 9) = "PXEClient" {
   next-server x.x.x.x;
   filename "pxelinux.0";
 }