IM domain aliases possible?

For various reasons, we are trying to setup a subdomain for IM. At the same time, we’d like this to be transparent to the outside world. For example, if I’m alex@im.x.com, we’d like for others to be able to add/see me as alex@x.com.

As far as I’m aware, currently whatever doesn’t match the IM domain is treated as a remote user. So here come the questions: is it possible to set up Openfire to do what we want? If not, is it possible to patch Openfire into doing that? Would that go against RFCs?

TIA