My experience with my Chargriller w/sfb and depending on the type of lump your using, sometimes that's enough without adding chunks, but I guess it all depends on how smokey you want your food. Just make sure you have plenty of extra fuel available. As time goes on you may even see some other mods you can to to help improve your chargriller. I for one built a charcoal basket in my firebox which allows me to empty the ashes during the cook and I create a "U" shape out of lava rocks and my coal, I light one end of the "U" and it snakes around giving a longer cook. Good luck with your first cook, looking forward to see to pics.
One last thing, If you do try the lava rock trick make sure you rinse them off and season them before using them on your smoke.
UDS, Chargriller Pro (retired), 22.5 Kettle, back yard with friends and family.