I think the Italians were going to convert a liner to a carrier, maybe the Japanese actually did it.
They did, both Junyo and Hiyo were based on liners specially designed to allow for such a conversion. Both ships were still undergoing construction in December 1941 when the order to complete them as carriers came down.
Not horrible ships, but their aviation complement was on the small side and, worse, their top speed kept them from being used as fleet carriers. They were more suited for independent operations as they couldn't operate with the faster ships.
Shinyo was a conversion of the German liner Scharnhorst which was in Japan at the time.
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German was planning to convert several liners including Gneisneau and Potsdam but these never got started IIRC.