You should be able to do it by just renaming the player file. You'll probably have some animation issues since DA has a slightly modified skeleton (I think) but it also might work AWESOMELY.
Go to this folder:
C:\Program Files\Steam\SteamApps\common\Double Action\dab\models\player
rename all the 'eightball' files to something like 'eightball_original' - make sure not to mess with those filename extensions (.sw.vtx, .phy etc). Then copy your model files in and rename them all to 'eightball'. Don't forget to copy the materials over as well, you won't need to rename them at all just make sure they are in the same directory as they were in gmod - create folders if you need to.
Then load up the game, select the character who is obviously not Nick from L4D2 and post screens back here!