I've smoked burgers almost every time I lit the smoker (for other meats). I just look at the grate and tell myself...there's room for something else...BURGERS!
For 1-2# I mix in:
1 tsp Weber Gourmet Burger Seasoning
1 tsp Worcestershire sauce
1/2 C Italian Bread Crumbs
1-2 tbs Honey BBQ Sauce
Mix and chill...
Naturally, all of the ingredient amounts can be tweaked, eliminated or substituted to your own tastes.
I smoke them to an IT of 160F. They have never been dried out.
Hot grilling (searing) for 1 minute each side (afterwards) is optional as grill/grate lines burn into meat when smoked.