Yes , it is definitely possible to add more charcoal to your smoker if it ran out of coals during the process. The process of adding charcoal can vary depending on the type of smoker you are using, but usually it is not much complicated. One thing to keep in mind is that adding new coals on top of the already burning coals can result in slight decrease in temperature for sometime, until the coals are ignited and start to burn . So the temperature will rise eventually. If you want to avoid such decrease in temperature to avoid improper cooking, you should consider igniting the coals beforehand. You can ignite the coals in a chimney starter and then place the coals the coal into the firebox.