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you should be able to modify the user menu template from XOOPS administration -> templates
you would have to 'clone' the default template set, modify whatever templates you need to modify (user menu would appear under the System module listings), and then in General Preferences, set the newly cloned template set as default.
just out of curiousity, why do you have multiple users logging in as one? that would then require all users to login with that one account (using that single username/password), which seems to almost lose the point of having a login/logout function.
i'm not trying to criticize, i just don't see the advantage of such an approach, and i'm hoping you'll enlighten me