What if the RN had captured the Altmark, I know if this had happened sh'ed probably end up staying as a fleet oiler. But lets say for some reason it was decided to convert her into an Escort Carrier. How feasible would it be.
Looking at the specs on Wiki, she was a good size (20,000 tons and nearl 180m long), with a reasonable turn of speed (21kts) so she could be a good basis for an escort carrier. HMS Audacity had similar origins (converted from the MV Hanover) and was significantly smaller and slower, so it sounds like a very reasonable idea. Another (quicker) option would be to turn her into a Merchant carrier, just adding on a flight deck and basic aircraft facilities rather than a full escort carrier conversion.
A "con" would be that such a large vessel would require drydocking at a large shipyard for conversion, and would possibly take up space and resources that were sorely needed for building new ships.
Altmark might have proved more useful as is, tanker and supply ship, to a fleet losing ships. The merchant carrier update might have given a useful extra capability if it was done so as to retain the original role.
JoeP