There are some guys who say they pulverized charcoal briquettes and mixed them in with wood chips or pellets or something and a couple who've used one charcoal briquette in the wood chip holder (I believe). Their reason for adding charcoal was to try to create a smoke ring in meat cooked in an electric smoker, which on its own is just about a chemical impossibility. I don't care about smoke rings so I never out charcoal in my MES 30.
You shouldn't need to ever use charcoal in an electric smoker to get a hickory flavor. You can easily buy hickory wood chips or pellets which work just fine. I only use wood pellets and I've got hickory, oak, pecan, apple, mesquite, and a mix of several woods. You can buy wood pellets and chips made in just about every wood "flavor" you can think of. Wood chips burn up very quickly but wood pellets provide hours of smoke if you set them up correctly. I use the A-MAZE-N Pellet Smoker and greatly enjoy it but there are other products out there.
You can try the charcoal briquette thing and see how it works out for you. Trader Joe's charcoal is really good stuff. I've got a Lowes closer to my home and I buy Stubbs charcoal there. It's outstanding without the additives and fillers Kingsford Blue contains and cheaper than Kingsford Competition. If Stubbs weren't available I'd be using TJ's.