I don't understand what you are asking. Moving threads is a xenforo function. It doesn't matter if a showcase item is associated with it as long as the owner of the item has permission to the forum you are moving the thread into.
You can edit a showcase item and associate and existing thread to it if you have the showcase moderator permission "Manage Thread Association". It is as easy as adding the Thread ID to the Showcase Item via the Item Edit function.
Is that what you wanted to know or were you asking if you can convert a thread into a showcase item? If its the latter, then no. You'd have to create a showcase item and then associate the existing thread to it.