You would need to query those fields in showflat and showthreaded, then add them into the templates.
For example.... in showflat.php, find this:
// -----------------------
// Cycle through the posts
$query = "
SELECT t1.B_Number,t2.U_Username,t1.B_Posted,t1.B_IP,t1.B_Subject....
Add a comma, then t2. then the field from the users table that you want to add to the end... like this:
...,t2.U_Extra1
Then below that, where it assigns them to a variable, add a variable for the new field(s) you pulled.
Find this:
list ($Number,$Username,$Posted,$IP,$Subject....
Add a comma, then the variable into the end of the list, like this:
...$usernum,$downloads,$anonname,extra1) = $dbh -> fetch_array($sth);
Then below it, add something like this:
$postrow[$i]['Extra1'] = $extra1;
Then in the showflat.tmpl file.... where you want the info to appear, (probably below location) add this:
{$postrow[$i]['Extra1']}
Hope that made sense and helps a bit.