Open the game console and enter add_army town nTownID, bUpgrade Will fill all hero army slots with 10 creatures of town id specified. Second param is 0 or 1 which controls whether creatures are from upgraded dwellings or not.

Town IDs:
HEAVEN = 0
PRESERVE = 1
ACADEMY = 2
DUNGEON = 3
NECRO = 4
INFERNO = 5

Example: add_army 5, 0 will fill all hero army slots with 10 basic inferno creatures. add_army 4, 1 will fill hero army slots with 10 upgraded Necro creatures.