it can be done but you will be sending unencrypted usernames and passwords across the network. i have done it with no problems.
in the register.php script look for :
Code:
mail ($newuseremail,$subject,$message,"From: \"$bbtitle Mailer\" <$webmasteremail>");
round about line 500
then after that but before the } insert the following code to connect to the other database, dump the details into it :
Code:
//Code added to enter details into another database
mysql_connect("serverip", "dbusername", "dbpassword");
mysql_select_db("databasename");
mysql_query("INSERT INTO put_your_table_here VALUES ('', '', '$password', '$username', '$email', '', '', '', '')");
//Code Finish
Obvioulsy, the insert into command needs customising to your table depending how many fields you have.
Hope this is what you needed .