The AirPort Extreme will not be compatible with the Orbi network unless you plan to connect the AirPort Extreme to one of the Orbi access points using a permanent, wired Ethernet cable connection.
Another way to say the same thing would be to state that the AirPort Extreme will not be able to connect to the Orbi network using only a wireless connection, if that is your goal.
In order to "extend" the network, the AirPort must be set up to "Create a wireless network" that uses the......
1) Exact same wireless network name that Orbi network is using
2) Same wireless network security setting
3) Same wireless network password setting
4) Be configured to operate in Bridge Mode
Even if you are able to meet all of the requirements above, how well your WiFi devices will switch from the Orbi network to the AirPort network when they move close to the AirPort Extreme is open to question.
You may find that you need to assign a different wireless network name to the AirPort Extreme network, and then manually log off of Orbi network and log on to the AirPort Extreme network when you want devices to connect to the AirPort Extreme. You won't have an "extended" network in this case, of course. You will have two separate wireless networks, requiring that you manually "switch" from one network to the other as needed.
If you want to try things out and need more tips on how to try to set this up, we need to know whether you will be using a Mac, a PC, or an iPhone / iPad to configure the AirPort Extreme.