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

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.