Custom dynamicphp top menu link

Hi!
I need to add a dynamic user’s button ( created through a php function ) on the top menu. This button must be seen only if the user meet some requirements ( example as cookie, is logged , etc. ). I can’t handle directly via WP or via the WP Menu cretor. This stuff are managed on a custom page template that work “outside” wordpress and make stuff outside WP.I can’t find where’s the display function menu and where to edit them. Can you help me find out?