Well, how handy are you? Depending upon the freezer you have, you're possibly looking at removing the interior plastic and insulation, then reinsulating, and reskinning. Then comes the heat source. Propane requires some plumbing knowledge and a BIG safety plan. Electric may require wiring and then you'll need a way to control the heat.
The above is for starters, and it may or may not be as complicated as I seem to have made it sound.
The charbroil side fire box addition may be easier, but some mods will have to be made.
It all boils down to what you want to do.