Change the link...instead of linking to
report.php, link to
Code:
private.php?s=$session[sessionhash]&action=newmessage&userid=x
This is where it gets hairy. If you don't have moderators, you can set
x to your user id. If you do use moderators, and want the reported post to go to them, someone else is gonna have to help you figure it out

as I can't quite figure how you'd do that.
Someone else will also have to help you figure out how to pass the URL of the reported post.
Hope this helps you get started