[Serusers] REGISTER from SER

Hello, I have a SER server, it has 2 phones registered to it. The phones are 300000 and 300100. The registration works fine, and I can place calls between them. I am calling this my 'PBX'. I have another SER server that can connect to a PSTN. I can register my phone number with it, and I can make and receive calls from the PSTN. Let's say that phone number is 2145551212(a)addaline.com. I'm calling this my 'GATEWAY'. OK, here is the tricky part. I would like to make the SER 'PBX' *REGISTER* 2145551212(a)addaline.com with the SER GATEWAY ... just like it is a UA. So an incoming call to 2145551212 would get delivered to the SER PBX, and I'll figure out which extension gets the call with local rules. For very simplistic delivery at the PBX I can alias 2145551212 as 300000. If the PBX crashes, or is turned off, the 2145551212 registration will eventually evaporate. Anyway, I can't figure out anyway to get SERPBX to register with SER GATEWAY. Is this possible? ---greg