You could try putting *123 into the SIP ID of that extension, and see if it can be dialled that way (have never tried using # or * as dialable digits there), other than that I'm not aware of another way of preceding the actual extension number with a non digit.
I believe what leejor suggested will not work as the IVR direct dialling is designed to search for existing extension at the set extension length.
I actually cannot think of a away to achieve the requires behaviour. Perhaps another forum member has more ideas.