I have done it twice. It involves buying either brass or aluminum inserts with the same female thread. I think it's 1/4" x 20. These are available at most hardware stores, just take the dummy adjuster with you. You can also find bushings for the unthreaded (normally top) portion.
I needed to bore the holes larger. In my case, I reversed the drill to prevent splitting the bridge foot. I just held it steady and took it slow. A drill press and small vice or clamp would help. Once everything is drilled and lined up, simply glue the inserts and bushings in with hide glue.
The piezo in a full circle is on the unthreaded side and it must fit perfectly against the wood in order to work so keep that in mind.
This is definately a job for a luthier unless you are handy and patient.
My luthier prefers cutting a new bridge because this method is difficult and there is danger of ruining a bridge.
You forget sometimes that you are playing music, not just playing jazz. ....Charlie Haden