Yep, you can re-use coals and/or wood. One caution, however: if you leave the coals in the WSM between cooks, they might tend to get damp if your weather is humid. That will affect the temp in your cooker, they won't burn as hot at the start.
I'm in Florida, so humidity is an issue. I re-use coals, but always mix in new charcoal with them. I only use new coals in the chimney, especially when using Minion Method.