have been super bummed about this and preparing to replay from the beginning cuz I like the game enough to do so but hallelujah I was able to take POPstarter’s .vmc save files, convert them into .mcd with MemcardRex and simply drop that file into Duckstation’s memcards folder (renaming the file to the default name Duckstation spits out when you start a game)
idk if any of this might be of help to you? MemcardRex can convert the .gme files that seem to be common on gamefaqs to Duckstation’s preferred .mcd