SASL and domain name

Hello. I have installed Openfire and successfully configured SASL using GSSAPI. But I want to make a little improvement to my installation.

My AD domain looks like subdomain.rootdomain.com, cause our office is filial of big company. So, according to manual, i should use (and am using) usernames like username@xmpp.subdomain.rootdomain.com. I want to make these names shorter, and ideally - make xmpp addresses same as email addresses, like username@subdomain.rootdomain.com. Can I perform this in any way?

Thanks a lot!

P.S. I have successfully created SPN for xmpp/subdomain.rootdomain.com, assigned it to service account and created and tested keystore, but that didn’t work, i cannot login using client application.