All you need to do is to configure an "init" eventhandler in your addon's config. From that EH, you then add the unit to an array. Keep in mind that an error in init EH's makes the eventhandler fire again whenever the unit gets into/out of a vehicle, so you need to make sure you only add the unit to the array once. If you need more info on how to config EHs, I'd post in the addon editing board, as suggested.