Adding another control to navigation bar

I don't know if I dreamed it or was real, but I recall seeing somewhere that it was possible to add a control to the navigation bar, but now cannot seem to find any reference to this.

Is is possible to add a control to the navigation bar? (I want to add a drop down box)
I am using pyGtk, can add to somewhere else in my form, .. but it makes logical sense if it is on the navigation bar.

Any suggestions?
Thanks
Steve