This is not possible. Cross save only enables you to access your characters on other platforms. All characters are bound to the platform they were initially created on.
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If they were made while cross save was active, where they were made does not matter. When cross save is active, you are accessing one platform's account on all your linked platforms. If you chose your PS account as your primary account for cross save, then you are always using that account. If you created a character while playing on PC, that is still a PS character, as it was made on your PS account. While cross save is active, you cannot change anything about your PC account.