it's two different machines for two different purposes. coffee grinders, both blade and burr do their job by cutting, so malt going thru both will have the husk chopped to crap, not good for sparging as you need intact husk to make a good filter bed. grain mills work by crushing, so the soft husk stays intact and the hard kernel gets broken.
for small batches of specialty grains, you could use a coffee grinder, but for all grain, it's not a good idea.