I hope somebody can help me with this.

I want to send a PM to all the users at my forum, about news and so, can anybody make a little hack that can do that?

Why would you want to PM all of your members when you can email all your members just as easily without hacking?

Because I want to be sure that all members get the message, cause I don't know if they are using their right email address...

But what if some of your members have selected not to receive emails from admins? If you end up finding the PM hack that you want, and you PM all of your members, what's the point? If a member selects not to receive email from admins, then that pretty much would suggest to me, that they also don't want to receive a PM from the Admin either....and that's assuming that they even have the Receive PMs option turned on. Regardless of whether or not they have a valid email address, if they choose not to receive PMs, your global PM isn't going to reach them either.