when i'm feeder fishing i always add brown crumb to my other flavoured groundbait(usually a bream or feeder mix). Mix each groundbait with each other 50:50, then add the goodies like your hemp etc...
i use hemp whole with my groundbait and it works fine, but then again i haven't really tried crushed hemp. i also add other stuff to it like sweetcorn, casters, and chopped worm.
extras like special flavourings, curry powder and mollasses etc... are an option to try as well.
BUT......... one of the main ingredients i never forget are OXO CUBES. these are brilliant. this bloke i saw fishing recomended them and the first time i tried it i had a good bream out. i just wish i could see him now and thank him.
some people may have already tried this but if you haven't DO IT!
and thats about it, i'm only 15 and is always trying out new things, but if you don't, you won't get anywhere unless someone tells you.
hope this is of some help. it certainly works for me!