Right.  If you want to extend your landing approach, add a little throttle.  That will level out the aircraft and extend the approach.  Then when ready, remove what you added and continue the approach.  "Altitude" may mean maintaining altitude rather than descending.  Same as Topgun and Jetmech said.  So, you are 30 feet up and coming in but you are short.  Add some throttle, the plane will level out (maintain altitude) until you are where you want to be, then remove what throttle you added and the plane will slow down and continue to descend. 

CGr.</p>