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

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.