Most composite stocks are a fiberglass, kevlar or carbon fiber shell, filled with expanding foam.
Inside some composite stocks will be hardware for mounting sling swivels, pillar bedding for some types of rifles, mounting points for hardware for adjustable combs, etc.
You can cut the fiberglass (or other) shell very easily; just use a fine-toothed saw. If you need to re-fill an area of the stock, you can use expanding foam to do so. If you need to patch a composite stock, you can buy fiberglass cloth and use runny Acra-glas as your epoxy/glue.
For what you want to do, you should make sure you have the mounting points for the recoil pad in place and glued or foamed in before you mount the pad, then grind the pad as you would on any other stock.