Spread cornbread cubes in a 12- by 17-inch roasting pan. Bake in a 350° regular or convection oven, stirring occasionally, until edges begin to brown, 25 to 35 minutes. Spread pecans in a 9-inch pie pan and bake in same oven until lightly browned, about 10 minutes.
In a 5- to 6-quart pan over medium-high heat, melt butter; add ham, onion, and celery and stir often until onion begins to brown, 7 to 10 minutes.
Squeeze excess liquid from mustard greens. Stir greens, corn, and sage into ham mixture. Mix in cornbread and enough broth to moisten lightly. Stir in pecans.
Spoon dressing into a shallow 3-quart baking dish and cover tightly.
Bake in a 325° regular or convection oven (350° if baking with a turkey at that temperature) until hot in the center, 15 to 25 minutes. Uncover and bake until top is lightly browned, 15 to 25 minutes longer. Add salt and pepper to taste.