Ello again 
You need to run a fake Hue bridge that the Harmony can pair to, the fake bridge can then talk to OpenHAB. People use similar methods to get Amazon Echo to send commands.
https://github.com/armzilla/amazon-echo-ha-bridge
https://github.com/bwssytems/ha-bridge
This thread is worth a read
(and you thought my Flirc method was long winded  )
 )
Depending on what you ultimately want to do with the buttons, it might be worth picking up a Hue Starter Kit. It is Prime day on Amazon today and starter kits (three colour bulbs and a bridge) are going for £99 (usually £150)
Bargain