@JimJim -
You cannot put anything new inside the ILS critical area of the airport, nor can you change the position of existing objects, without causing the existing ILS to become uncertified. You cannot even have equipment, vehicles, personnel, or other aircraft in the critical area. About the WORST thing you could possibly place in the critical area would be a non-transmitting antenna tuned to the same frequency as the existing transmitting antenna. This can set up multipath distortion leading a false glideslope indication.
Prior to the use of the new glideslope for navigation, it needs to flight-checked and certified by the FAA.
FAA updating flight inspection fleet for a NextGen world - Aircraft Owners and Pilots Association