Never tried it, but it won't turn out well even if the game could technically load it. You'll probably have missing or incorrect item lists and NPCs, (like Borcha being king or Harlaus wearing a wooden spear as a helmet), and other similar bugs, because of inconsistencies in their item_kinds1.txt and troops.txt files, which are some of the main differences between the two.
Instead of that, to be safe, just use the export/import character function of Warband. You'll lose all your progress as far as your kingdom and party is concerned, but you get to keep your character's level and gold, and you can use that to just start a new party and kingdom. Think of it as if you defeated and captured by bandits, etc.