yes, use a document reelative path.
for example if the page that sends the email is at:
http://mysite.com/pages/sendEmail.php
and you want to attach file that is one directory bellow the site root, you would need to move down 3 directories, so:
$WA_MailObject = WAUE_AddAttachment($WA_MailObject,"../../../folderName/filename.pdf");