Try it with a minelayer. A minelaying sub trades in its torpedoes, (don't laugh now) it may get supply "credit" for that.And I looked closely the last time and saw no supply being consumed for mine loading (on a minelaying sub, in my case).
Only if it is at a friendly base. You can see it on the base information screen. Maybe top center?How can you see how many mines were in a friendly mine field? I didn't know this could be done.