In a 5-qt. slow cooker, whisk together beer and 1/2 cup mustard. Stir in pepper. Add ham and rosemary. Cook, covered, on low until tender, 7-9 hours., Remove ham; cool slightly. Discard rosemary sprigs. Skim fat. When ham is cool enough to handle, shred meat with 2 forks. Discard bone. Return to slow cooker; heat through., Using tongs, place shredded ham on pretzel buns; top with remaining mustard and, if desired, dill pickle slices., Freeze option: Freeze cooled ham mixture in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a covered saucepan, stirring gently; add water if necessary.