I'm not sure if there are any such plugin that could help you in regards to "non-users". However, if your purpose of having a searchable non-user is to ensure that they are searchable but not counted as licensed user, you could do this by creating the users, but not providing them any privilege or membership.
As mentioned on our documentation here, Confluence will only count the user as licensed user if it have "Can-Use" permission.
Hope the above information helps you on this.
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