It would require hand coding to do this, since powermessenger uses php mail() by default, which cannot send email using an API or authenticated mail.
The easiest solution is probably to use PEAR if you can get it installed on your server. That way you could do authenticated mail through SMTP without using their API, which would be a lot more hand coding.
I think if you got PEAR installed you would be able to update the /webassist/email/mail_php.php file with the PEAR version, which I've attached. Then the only lines of code that would have to be hand edited would be the 6 files that begin with waue_ in the webassist/email folder.
You would just have to update line 15 of each of those files from:
$WA_MailObject = WAUE_Definition("".($WAGLOBAL_Email_Server) ."","25","","",$WAGLOBAL_Email_Use_Additional_Params,"utf-8");
$WA_MailObject = WAUE_Definition("","".($WAGLOBAL_Email_Server) ."","25","serverUsername","serverPassword","true","","","utf-8");
and put in the username and password for the remote SMTP server to replace the placeholders I've added.
This is something we could help with in a premier support ticket if you needed assistance.