Every MLX and TDL station has a "second" extension number, for an adjunct (which is a card that can be installed into an MLX phone to provide a S/L station or an alert). Since no one actually uses the adjunct devices, and since you can't even add one to a TDL phone, they make great phantom extension numbers. By default, if your first phone had extension 7100 in set upspace, the adjunct extension is 7300. If your first extension is 100 in 3 digit numbering, the adjunct is 300.
So they are already there, you can assign lines to them, give them voice mail coverage, and renumber them just like real extensions.