It depends on how your system is setup. Do your DID's match your extension numbers exactly? As in you only see 4 digits come into your system from the carrier, so they have to route directly to extensions? Or do you use the System Speed Dials form to route DID's to extensions?
In the first case, it becomes slightly complicated. You have to create a phantom hunt group, and then using call rerouting, route said hunt group to the primary extension number.
If its the second case, then all you need to do is program a speed dial of the DID and for the actual number part, put in the extension you want the DID to ring on.