Scripting help to populate proxyAddress in AD

Hi guys, I could use your help with this..

I have an AD in which I need to populate every user´s proxyAddress in a specific OU using the city attribute (already populated). So every user should have something like this in the proxyAddress attrib:

SMTP: "city@somedomain.com"

SIP: "city@somedomain.com"

Any idea on how to build it?

Thanks in advance!