On the other realism hand, it didn't work that way. Unless you were very close to home port they wouldn't respond to an SOS. There were resupply subs ("milk cows") but they only became available in March 1942 and the last one was sunk in June 1944. They were sent to specific locations and not available for 'SOS' work.
http://www.uboat.net/types/milkcows.htm
On the other hand Trevally's idea is a good substitute. If the game doesn't have something that was there, it's not cheating to put it there, if only for one trip home.