Welcome to the forum, Strand!
Go to your documments/SH3/Careers/your name/
and open the file Patrol_0.CFG with any text editor.
Scroll down to the end of the file, and in one of the last lines, you'll find something like "Promotions = 0"
Change the value "0" to any value you like. Load your game back and you'll have that amount of promotions to distribute.
If you want to manage your crew outside the game in a very cool and easy manner, I suggest you install SH3 Commander, which is an awesome software. Will allow you to modify the game into a much more realistic and enjoyable way, like randomizing ship tonnage and uboat crush depth.