Actually, you need the Pro version of Unity to do any type of modding for Desperate Gods. David employed some custom shaders or some-such advanced tweaks that aren't supported by the free version.
On the flip, you can search for Vonkie's Mod on this forum and download her most recent version (Windows Only).
Vonkie hasn't updated her mod in quite a while (probably moved on to bigger and better things), but even the current version allows for a lot of options for modification.
Once you get into Vonkie's Mod you can add more cards into a pile, and adjust what their images and values are. I don't think you can actually add a new deck pile, but you could re-arrange things to look the way you want using the existing piles.