Log into the Poptropica account you want to change the costume of. Make sure you are on a non-SUI (non sound updated island).
Open the Poptropica Avatar Studio in another tab.
Type minecraft:steve in the username box.
Refresh Poptropica. If a message appears, do not hit retry, hit cancel. Press ‘Load’ a bunch of times on the Avatar Studio tab, then go back to the Poptropica tab and you’ll be prompted to save a new account with the cloned costume. Note that this will replace your current costume with that of the user you typed in.