You can create your own copy of the Observatories table by taking it from the "<CASA-distro>/data/geodetic" directory in your CASA installation and then editing your copy with the browsetable task in CASA. Once you have your own copy, e.g. in the directory "/home/myself/mydir", you should create or (if it exists) edit the file "~/.casarc" and add the line
This will tell CASA to pick up your copy of the Observatories table instead of the default.
Note: The table must still be named "Observatories".
More information can be found in the CASAGuides.