I am going to guess you could only get say 21 burgers on the cooker at a time (figuring 18x36 cooking area and 5" round premades) so doing 100 of them is going to take a while unless you have multiple cooking racks in your cooker.
I have never done big burger cooks on the pellet cooker but have on a gas grill. I will cook them to where they are not completely done and shrunken up, then they get transferred to an electric roaster or a chafer to keep warm, in the roaster or what ever you use I have beef broth, some seasoning garlic, onion, black pepper and sometimes some chili powder, also some Worcestershire sauce and some tequila. If you can adjust the temps on a roaster set it at 160 ish and just allow the burgers to to finish cooking in the liquid they will stay juicy and heck I have been know to add a touch of the liquid smoke in there also.
Good luck to ya on your big cook.