You need to use this filter to add an additional meta field.
add_filter( 'wpsl_store_meta', 'custom_store_meta', 10, 2 );
function custom_store_meta( $store_meta, $store_id ) {
$store_meta['mi'] = 'km / 1.609 or';
return $store_meta;
}
Next use this filter to include the mi value in the search results template.
Just replace ‘my_textinput’ with ‘mi’, and the value should show up. Do make sure to flush the wpsl transient cache as well on the WPSL settings page ( tools section ).