What happen to email that is forwarded when the junk mail is active for this particular domain? Does the junk mail filter act on them? If it does, how can I access the webmail, without a proper account on it?
Your forwarding addresses don't get Junk Filtered. And if it's forwarded to another account on DreamHost, it doesn't even get filtered on its way into your In box because it came from DreamHost.
So, no, there's nothing you can do.
Can I make it at least filter it on the way of my other acount? No way, not even a "hard" with .promailrc?
I don't use the .procmailrc approach, but if the mail is to your domain user account (not an mXXXXXX account), you probably have a chance.
I have set up to forward postmail@* to my domain mail account, and it looks like .procmailrc works here. At least, when I recieved my test message it was checked by my SpamAssassin.