Except for that insane I400 class. It if had stopped carrying planes and started carrying bullets, it may have been JAP's answer to their freighters getting splattered. Could even have fired a torpedo or two at a convoy after it had dropped off its cargo and was on the way back ;p.
No not really.
One I-400 could carry 3 x 4 ton planes, so optimistically perhaps 20-30 tons of Cargo extra for removing the planes, and only 3 of these subs were completed near the end of the war for a large cost each.
When Japan entered the war they had a total capacity to carry around 3 million tons of Cargo using their entire merchant marine, normally at least 500-1000 ton or more on each ship. And this was hardly enough to supply all their forces at the height of their expansion even without accounting for losses.
Some numbers on the supply needs of forces:
German armored division:
30 tons per day when inactive
700 tons a day when in heavy fighting
German Infantry divisions:
80 tons per day when inactive
1,100 tons a day in heavy fighting
This meant that even all the three I-400 submarines built had to be able to make a trip each day to be able to supply an inactive Infantry division, so it could not be stationed much further away then 200-300km, or else the submarines won't have enough time to make the trip.
