30 5gal bucket w/ ground nutshell and cedar fiber??

Ok so starting a new grow and trying to get enough soil mix to fill 30-40 5gal is proving to be a bit more tough than I expected. My question is can I mix ground nutshell and or cedar fifer into my soil mix to get a bit more volume?? Or would I have to compost it?

I have a lot of 5 gal buckets with soil, but when I went to fill 50 gal barrels, I realized real quickly I couldn't afford to buy or physically/mentally haul all that soil a 1/2 mile into the woods, so I filled the bottom half of the barrels with the natural leaf/pine needle/dirt topsoil from the grow area. On top of that I started mixing my soil mix in with the detritus, and then the last 1/4 of the barrel was the really good soil mix of organic bagged potting soil, ash, compost, manure, etc. I have had zero problems. I think it is probably better for doing this.