That would be a bit hard to execute. You'd need a way to split the nade, propel the spinning part, and still have room for bb's and the feed mechanisms. It would probably have to be fairly large, wich would mean it would be heavy. If you look at this site nder pyrotechnics, you can see an airsoft grenade that is explosive powered.
www.tea-and-medals.co.ukA way to do it here would be to replce the explosive with a gas chamber with a bunch of nozzles. The bb shooter would be colored bright orange or a similar color, and you would have replacable grenade *shells* that would go over it.
Still complex. You could also remove bb's entirely from the above setup, and just have the gas rupture the *shell*. The shell would be like paper mache or somthing, thin and no more hazardous than a bb.