There are a number of ways you can do this, I’m happy to suggest a few but understand that everyone will have their own take on it.
If you want a two way solution (if you have some spare zone inputs) you could look at a Robot Electronics IO board that would give you UDP or TCP over IP control?
Like this unit for ~£70 that offers
4 x Relay outputs
4 x analogue inputs
8 x digital IO
There is a box available too.
They have a DScript version if you want to add some extra logic.
Alternatively, you could add a couple of Velbus modules which can be linked to openHAB2 via USB.
VMB8PBU or VMB6PBN for picking up the contact closers.
VMB1RYNOS for triggering a zone input.
VMB1USB for interfacing directly to openHAB2.
You could probably grab 60ma of 12Vdc from the alarm to power these Velbus modules.