Control Panel » Primary Settings » Advanced Options
look for: Main Administrator User Id
That should be the board owners ID number and who is sending the welcome message PM.
This will change the Master admin for the board so all messages of any type will come from what member number you change it to.
For just the welcome PM only.
Quick down and dirty going from memory and not tested.
Edit adduser.inc.php
change ($config [main_admin_id])
to
member number of your choice
I beleive there are two lines to change.
Remember this is considered a modification and is not supported here.