5 digit dialing t o external auto attendant?

We are in the process of moving to RingCentral for several of our users (new company subsidiary). We would like to be able to use 5 digit dialing to call users on old system. The old system has an auto attendant so we were hoping to be able to dial something like 2-1234 wher 2 would initiate the call to the old auto attendant and then pass the 1234 as the extension. Is this possible? Is there some other way to achieve this ?