However, if you sign up for OnSip.com's A La Carte Account (Free....but careful don't mix this up with the Free 30 Day Trial), you can do just that, create any form of
name@mydomain.com SIP URIs.
You do have to be able to access your domain's SRV record to do this....if your current domain registrar doesn't allow you, GoDaddy's OffSite DNS supports this and is free, simply set up an account at GoDaddy...
You then have the choice of forwarding the custom SIP URIs to any other SIP URI (eg. VoXalot), or registering with them directly...(but there is little point to this unless you are planning on using OnSip for outgoing calls)