Not familiar with deskcnc, but most CNC software will allow you to add offset.
The easiest way may be to export the file to LazyCam (you can do that from Mach3) and then add a 1/8 inch offset. You may need a LC pro license to do offsets.
If that is not an option look at CADX11
http://www.graytechsoftware.com/ They have a free version of CADX11 that will do what you want.
Several fairly inexpensive ways to do it. Shoot me a message if you need more info.
Bob@BobsShop