Can I add or change available subs in the Single Missions?

[CapnScurvy]

As far as your original question....... "Can I add or change available subs in the Single Missions?" Yes you can, within limits. Next to the sub class that's the "default" sub for the mission you'll find an "arrow". Clicking the arrow will being up a list of available subs for the mission. The limiting factor is the date of the mission. Only the sub classes that were available at the time of the mission will be available........ie. don't expect to find a Balao in a 1942 mission. The game is designed to be historically accurate in most cases, this is one of them.