The easiest way to do this is to create a WinPE configuration named something like "BartPE Boot" and replace the ...\eXpress\Deployment Server\PXE\MasterImages\MenuOption<XXX>\X86PC\winpe.iso with your BartPE.iso file renamed to "winpe.iso".
Note: Instead of renaming BartPE.iso to winpe.iso, you could also edit the PXE configuration file called "winnt.sif" to reflect the name you wish to use for the .iso.
Another way to do this would be to create a new PXE boot option using a name such as "BartPE Boot", choose Other as the operating system and User supplied as the Image Creation Method. When you click on the Manual Boot Image button, it will instruct you to copy PXE files to a new directory. Again you could copy the necessary files from a WinPE configuration replacing the winpe.iso file with your "BartPE.iso" file.