I got some saw dust from the local Box store. It was pretty chunky so I bought a cheap blender (barely smart enough not to use my wife's kitchen one) and tried to cut it down. Didn't work in the dry. I slurry it up, but that didn't really work well either. I have a fruit drier that I was going to use to dry it out.
WPC (Wood Plastic Composite) is a really tough nut to crack. The commercial stuff is Polyolefins for decking and PVC for windows and getting the wood to compatibilized with the polymers isn't easy. Then add in the wood types out there... Getting it dry is critical.
I assume we want to use standard 0.4mm nozzles, though I have a Printrbot Plus that I have a 0.8mm nozzle on. A 3mm filament machine with a 1.0mm+ nozzle is what I'm thinking might work best for home made filament.
I've looked at trying to find finer wood particles. Rockler has wood flour listed. Maybe a marine supply place might have some for mixing with epoxy for repairs.
I've started looking at conductives, so wood is on the back burner.
Don't forget to take out the melt filter if you get that far.