Actually threads doesn't have the ability to define the cookie domain. The setting in the config file is for the referrer check.
This would require some modification.
Untested - make backups first.
But in ubbt.inc.php you'll find the ubbt_setcookie function.
This line:
setcookie("$name","$value",$time,$cookiepath);
Would need to look something like this:
setcookie("$name","$value",$time,$cookiepath,".yourdomain.com");
Again, untested - and use caution when you modify. <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" /> Modify at your own peril. <img src="https://www.ubbcentral.com/boards/images/graemlins/wink.gif" alt="" />