[Serusers] Local and external access

Is it possible, in an office type synario, to have SER be the 'exchange' for local VoIP clients but for it to also register a user into an external SIP router? In case that question is not clear: I have an account with a VoIP provider where if I get my VoIP client to log into their router I can make & receive POTS calls - so people can call me on a land-line number and it arrives at my SIP phone. But I want to be able to make calls to other VoIP phones in my office area so I want to log my SIP phone into our SIP router and not the external POTS providers SIP router. I don't want my local VoIP calls to depend on my Internet link. So what I really want is for my local SER to register me with the external SIP router when I my SIP phone registers with my local SER. Is this possible? -- Mark Himsley