What series of 18?
The 18A uses an Eprom to store the ladder and you would also need a reader to read and then re-burn an Eprom. The 18B and 18i series went to flash rom.
If it is an i series the onboard edit function may be available depending on what options are included in the control (c language).
In addition to fapt ladder there is an edit card (PMCIA card) that can be used to edit ladders on the B and i series.