Might be helpful to identify your mill, just incase it is not one size fits all.
I have a cereal killer that I power with a drill. I had two batteries go flat during my last milling session and had worried about having to dig out the hand crank but luckily the last battery worked.
I have had issues with dumping in grain then starting the drill where the mill does not pull grain thru the mill but just flows over the free spinning roller. I solved the problem by easing grain into the mill with drilling running them pour in the rest once the grain grabs. I am not sure if this is an issue with using a hand crank but I dont think I could hand crank and feed grain at the same time(could be a personal problem).
If you hard set on a hand crank might be able to build one out of wood or McGilvery something with a vise grip or a wrench.