[openbox] logging as another user

[Piscium]

With plain Gnome or KDE it is possible to do this: while logged as
user A, to log in as user B, and later return to the A session.

I use plain Openbox (that is not gnome-openbox nor kde-openbox). Is it
possible to log in as user B while keeping logged in as user A?

Currently I have this in my menu.xml:
  <item label="Log out">
    <action name="SessionLogout">
      <prompt>yes</prompt>
    </action>
  </item>

If it is possible to login as another user, what should I put in the
menu configuration?