Need help with Jobs & Experts

Right now Experts have the permalink structure /expert/firstname-lastname...this is because the title of the Experts posts is "firstname lastname"...how can I change the permalink structure to a custom field instead of the default "firstname-lastname"...is this done in the custom post registration?

I'd like to change the permalink structure for Experts from /firstname-lastname to a value based on a custom field.

How can I do that? Please help!