Is it possible to have multisite exclude a domain?

I've sadly had to give up on Buddypress - its just doesn't quite work as intended... and when speaking with the buddypress folks I'm getting one word answers... they're not nearly as helpful as on here.

The long and the short of it is - I've had to use Elgg.... It was really quick to install... paid for a theme... job done... does everything out the box....

Only thing is, I'd like to keep wordpress multisite for the website side of things.. Is there any way for me to exclude one subdomain, and set Elgg up on that?

so I can still have both powered by wordpress

and... then a seperate domain... for elgg?