Welcome to our newest members appears in the ubbthreads.php language file as:
$ubbt_lang['NEW_MEM'] = "Welcome to our newest member,";
and the catagories.php language file as:
$ubbt_lang['NEW_MEM'] = "Welcome to our newest member,";
So, you should be able to enter the text you want to above
{$ubbt_lang['NEW_MEM'] in ubbthreads.tmpl and language.tmpl and put a <br /> after your text.
If that doesn't work, as someone else had problems getting a hard coded disclaimer to show up in posts, you may need to define it as a varible in the language files and call the varible.