Reading in the manual, it says "if you have opted for the Clifford Dataport interface"

How can I tell if the original owner had the port fitted or can I visually check if the alarm has the port?

Is it something that can be fitted to alarms already installed?

Check from the main control unit for a white 3 pin connector on the end of a short black cable. If that is connected to anything then it will probably be a data port extension.

My previous car was a JDM celica, i had the clifford concept 600 with the data port fitted. the data port can be fitted any where, i had it fitted on the left hand side of the centre consol it cost £50 extra. If fitted you will see a small adapter around that area. It allows you to alter the settings of the alarm via a laptop.