It can be done, because I've done it. Not proud of it, but it was many years ago when I was in college. Brother was doing two for ones on a a ten year stretch in a medium security prison. Day before I'd grind up the weed to a pretty fine consistency, then stuff it into small balloons, double wrap it.
Having visited many time they get to know you, and you get the feel of the place. Always just kept in my back pocket, and when waiting in the visiting area I'd buy a pack of peanut m&m's and a few other things, plus water and soda's. Then I'd take them from my back pocket to my shirt pocket then start dropping them in the m&m packets. Looking at them you couldn't tell the difference. My brother used to say that the C/O's probably knew but would rather have inmates sitting around stoned instead looking for trouble.
[Moto Creed-O: Take the High-Road.....Kick His Ass and Fuck His Chick