I don't think it is that simple though. Taking a non-Navy ship and trying to convert it into something like a carrier or battleship you run into problems like weight distribution and speed. Purpose-built carriers are designed with the center of gravity very low so as to prevent the ship from tipping over due to the weight of the stuff on top. Oil tankers have the weight distribution specially designed too, but for carrying oil. If the oil tanks are empty, and then you add a bunch of stuff on top, that might change things around in a non-ideal way.
Then there is the issue of speed. Aircraft carriers are much faster than oil tankers, which move more slowly. So to convert an oil tanker to a carrier, if you want it to be able to move fast and keep up with the other purpose-built naval vessels, you're probably going to need to change the propulsion system as well.