Sorry, but I very much doubt the The Archers (or any other radio programme for that matter) ever uses 'extras'.
The convention with radio drama is that any voice parts which don't justify employing another actor are played by members of the cast. When the script calls for background voices, such as people at the county show or the village fete, these are provided from sound effects recordings.
You'll just have to get famous, like Alan Titchmarsh, Stephen Fry, Princess Margaret or Mike Gatting, to name but four celebrity 'extras' who have appeared in Ambridge.