No. Once you finish the game it is over, at least until the
add-on is released. The add-on continues from the end of the main game, so playing them together it would be seamless, and you can play with your current character (keep a save near the end of the game) or start a new one.
If you want to avoid spoilers (discussion of the add-on will eventually mention details about the end of the game), once you enter the hall of echoes you can not get back to any other area, so delay doing so until you have completed everything, enchanted all your best equipment, etc.
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