What you can do is copy the "signup-xtra.php" file, included in the ZIP in the "xtra" folder, to your main ListMail folder.
When copied to the main folder this script will run after every signup and, optionally via checkbox, after manual additions to your list.
In this file you can make a new database connection, etc. You'll want to make sure the commands only work during a signup to a certain list with code such as follows:
// run only if subscribing to list 2
if($list=='2'){
$link2=mysql_connect('lmhost','lmuser','lmpass');
mysql_select_db('lmdatabase');
// create a unique ID..
$uniq = '';
while(!$uniq){
if($randmaxx) $randmax = $randmaxx;
$uniq_str = strtolower(substr(md5(rand(0,$randmax)), 0,7));
$ucmd = "select id from lm_users where uid = '$uniq_str'";
$urow = mysql_query($ucmd,$link2);
if(@mysql_num_rows($urow)==0) $uniq=1;
}
// insert the user with all the same data as the user signed up with (provided to signup-xtra.php automatically by ListMail)
mysql_query("insert into lm_users (fname,lname,email,dateadd,uid,refurl,ipaddr,cnf) values('$fname','$lname','$email','$today','$uniq_str','$refurl','$ipaddr','1')",$link2);
}
// all done
Note, do not use the "exit()" function after your code, you must simply restrict the operation on a per-list basis by using an IF statement.