U-Boat Missions Add-On used to be sold by Ubisoft, but they no longer do. The Steam version is only for a Steam "base" game of Wolves of the Pacific, which is a v1.3 install. If you try a Ubisoft v1.4 on a Steam v1.3, you will get an error message that the game has already been updated. So you have to keep the Steam versions together, and the Ubisoft versions together, other than they no longer sell the Add-On, and only have "Wolves of the Pacific - Gold Edition", which is less expensive anyway, than what the Add-On used to be... lol
Silent Hunter 4 - DLC - PC (Digital) on the Ubisoft US Store