Transferring a torpedo from the rear below-deck canister to the front loading hatch would have been as or more difficult than transferring a torpedo from a supply ship.
IIRC, transferring a torpedo from the supply ship required strapping numerous life jackets around the torpedo and floating it to a position above the partially submerged bow of the u-boat. The bow is raised and the torpedo is manhandled into and down the front loading hatch.
Transferring torpedos was a dangerous, time consumming activity. As the war progressed there were fewer areas where this could be safely accomplished and many boats sailed without external torpedos. In the mid to later stages of the war, the u-boats (primarily Type IXc and Type IXd2) that were allowed external torpedos sailed only to remote areas; Indian Ocean, South Atlantic, etc. These were the only ones that had any chance of safely transferring externals to the inside. "Darn those airplanes!"
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