I really don't know if you can actually join it to a domain. I have not found anything on actually going through the proccess, and I have never tried.
What OS version are the Macs running?
Here are a few interesting web pages I found.
Link1
Link2
Another thing you can try... I've never used Outlook for Mac, but on the PC version, if the computer is not a member of the domain that the exchange server sits on, you can do this.
Go to your mail properties. Under the properties for the Exchange Server, there is a setting that you can choose for "Logon network security". Try setting it to None. Now, instead of trying to pass on any password/security info, it will just bring up a box with username, password and domain name boxes.
I have run into problems with PC's in one Domain, trying to connect directly to a exchange server in another domain. This will takes care of the problem in my situation. May not be a possibility for you, but it's worth a shot.