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

You have to consider a couple of items:
Do you want to use the crossover in the subwoofer or run everything full range.  Right now, you could either use the line level from your pre into the sub, then to the power amp.  OR you can use the speaker level outs from the crown to the sub to the speakers.
The other options are:  You can use a Y cable from your pre to the amp and sub to avoid the crossover.  OR you could use a sub like REL that has speaker level taps that won't interfere with the signal at all.
Personally, I would like to ask if you have tried a home audio amp vs the crown?  Also, what is your pre amp?  Commercial and home audio can be quite different-IN MY OPINION

Okay, I can see some areas for confusion here that will require clarification. Which scenario are you asking about:

1. Use the Crown amp for main left/right speakers and also connect it to a subwoofer with built-in amp. If this is the case, then elevick’s comments apply.

1. Use the power in the Crown amp to power/drive a subwoofer speaker. If this is what you want to do, then you need what is called a "passive subwoofer". It is, essentially, just a woofer in a box with speaker wire connections. They are out there, but they are very rare - as most people will want "powered subwoofers".

There is a passive subwoofer on audiogon:
https://www.audiogon.com/listings/lis9g48b-tannoy-definition-sub-15i-15-passive-subwoofer-19501-subwoofers

Parts Express will sell passive subwoofer kits if you want to build your own.
https://www.parts-express.com/cat/speaker-subwoofer-kits/286?N=23647+4294967118+4294965883&Ne=10166&Nrs=collection%28%29%2Frecord%5Bendeca%3Amatches%28.%2C%22P_PortalID%22%2C%221%22%29+and+endeca%3Amatches%28.%2C%22P_Searchable%22%2C%221%22%29%5D&PortalID=1

You can search ebay for "passive subwoofer".

For this passive subwoofer idea to work, you will need some sort of active crossover or HT Processor/receiver that will only send bass signals to the Crown / passive subwoofer.

will be used as an integrated amp to drive Magnepan LRS'
the amp actually has built dsp crossover functionality

Hmmm.  Yes, this amp, as well as several other Crown amps, do have DSP.  However, this is only a two channel amp.  If you look at page 8 of the manual, you will see that one amp can be configured to do a crossover between Output Channel 1 and Output Channel 2.  Channel 1 can be configured as low-pass for the subwoofer, and Channel 2 can be configured as high-pass for your main speaker.  However, you are only powering one side of the room.  To do what you really want to do, you will need two XLS-1002 amps.

huh? can I not share the speaker level connections with either an REL sub or another sub that accepts high level?

will be used as an integrated amp to drive Magnepan LRS'
the amp actually has built dsp crossover functionality

Well, yes, but your comment above seems to indicate that you wanted to use the Crown to "power" the sub.  You didn't really explain what you were trying to do.

huh? can I not share the speaker level connections with either an REL sub or another sub that accepts high level?

Yes. You can. Sorry, don’t know why people are making this so hard for you. If the sub accepts line level and you have a line out then use line level. If no line out and the sub accepts speaker level then use speaker level. Simple.

So no worries. Hook it up. Enjoy.

sorry, I can see how my post might read funny
I've never done a speaker level connection from amp to sub
I've read of people with tube amps using different taps, and guys with a/b sections on stereo receivers using the second set of speaker terminals
a little confused on how to share the same set of terminals

should I use spade connections to share with the current bannana terminals?  never used anything but bannanas before so a little worried about screwing this up

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.