Well, first of all you need to have your phone rooted.
There are a lot of good tutos for that on the web. Personnaly I used the Nexus Toolkit to root my GNex and Nexus 7.
Here is a tuto : http://www.wugfresh.com/dev/nexus-root-toolkit/
Once done, you have to install an alternate bootloader to allow you to install kernels and roms.
You can use ROM Manager to install CWM 22.214.171.124, the best bootloader. https://play.google.com/store/apps/details?id=com.koushikdutta.rommanager
After that, download GlaDos kernel at this address : http://goo.im/devs/Ezekeel/GLaDOS-GalaxyNexus
Copy the zip file on your GNex and reboot in recovery mode.
Install the zip, the kernel is installed !
Now, you need a terminal app to run commands.
I use Android terminal Emulator. https://play.google.com/store/apps/details?id=jackpal.androidterm
Once installed, lauch it.
To switch to root user type :
To load usb audio modules type :
To change the value type :
setprop media.audio.out.mode usb
Now your GNex should be ready to use USB DAC.
I've finally ordered for a E7 DAC. It is a self powered DAC so I hope it will work with my GNEx.
I keep you updated !