is it possible to connect a subwoofer to Crown XLS amps?

Audiocanada, you sure can and you only need a passive sub as you have full crossover capabilities in the Crown. If the sub has a plate amp in it just remove the plate and identify the wires going to the driver. These are the ones you will connect to the amp. You should be able to identify  the + wire. If not don't worry about it as you can always reverse phase any time you want. I would solder 14 gauge speaker wire to these wires and cover the connection with heat shrink tubing. If there is no hole in the plate amp to lead the wires out I would drill a hole just large enough to fit the wire. Make a knot in the wire just were it will run through the hole so you can't pull it through. Run the wire through the hole then inject silicone caulking into the hole around the wire. That will prevent any leaks.
Now you are in business. Program your cross over and Rock! Forget about the speaker level connections just bypass the whole mess!

again, amp will be used as an integrated to power passive speakers, sub would be self powered
2.1 stereo system

A picture worth 1000 words

Speaker level connection:

https://www.audiocircle.com/image.php?id=120343

Line level connection: 

https://community.klipsch.com/uploads/monthly_2017_01/5880223749318_ScreenShot2017-01-18at8_19_10PM....

My error. If the sub does not have speaker level inputs you have a problem. You do not have a preamp output on the amp. If your source has one you could use that as long as the volume is controlled at the source. It is an unusual way to do things. You might consider using a preamp. 

The Crown has an line level loop out of the 1/4" connectors.
You'll just need to buy RCA to 1/4" adapters and then run the line level out to you sub.

Done.