WHY HELLO THAR! I'M YOUR FRIEND RIGHT NOW!
Now that we're improperly introduced, here's how to do it.
To put a new model on an old rig and animation set, you have to borrow the old model's bones. You can achieve this by opening up the decompiled original.
To decompile the zombie model, you can decompiler the model using a tool in Milkshape3D or an external one like this
http://www.wunderboy.org/download.php?file=mdldec12.zip&s=2 Either way, once the skeleton is in milkshape, load your model and bind the new model to the old skeleton. May take some tweaking but its never too bad. To rig in Milkshape, there are a million tutorials on youtube.
Export the model using milkshape's export tools and put the mdl file in your models folder for HL. The model will retain all of the animations but sport a shiny new appearance.
There's also should be a note from me in your inbox.
Have Fun!