If I read you right you're looking for the "From existing publication..."
option which appeared on the New Publication Task Pane and allowed for
creating a new file from an existing file as though it were a template.
Unfortunately, 2007 has revamped the Task Panes & moved the new pub options
to the Getting Started Window (which has *more* than enough available space
to include the option but *doesn't*) so that makes for a new can of worms
Go into the Open File dialog & navigate to the desired pub file, Right-Click
it & choose New from the contextual menu.
You might also consider using the Help menu>Contact Us to send feedback to
MS if you would like to have the feature "relocated" to a more convenient
spot.