In Dnn DotNetNuke.Entities.Users.UserController.GetPassword(ref UserInfo, "") is used to get the User password

use the below  code to achieve this

string DonorPassword = string.Empty;
                DotNetNuke.Entities.Users.UserInfo UserData = new DotNetNuke.Entities.Users.UserInfo();
                UserData = DotNetNuke.Entities.Users.UserController.GetUserByName(PortalId, UserName);
                DonorPassword = DotNetNuke.Entities.Users.UserController.GetPassword(ref UserData, "");
                return DonorPassword;

Hope it helps.

you cannot decrypt code in database, you either need a custom module with the code above or use a 3rd party module with export features, like PHD BulkUserManager (don't beat me on the name) you may buy at www.snowcovered.com.