Add your own JOIN to the parent query of SearchWP's search algorithm

gistfile1.php

<?php
 
function my_searchwp_query_main_join( $sql, $engine ) {
  // modify $sql to your liking
  return $sql;
}
 
add_filter( 'searchwp_query_main_join', 'my_searchwp_query_main_join', 10, 2 );