RoundCube can't support client side filtering, ever time a client login it wound have to sort all of the new emails into folders. It would make the server use up to many resources. It has to be done at the server lever as the emails come it, however I am confused as to what you are trying to do. You have Spam Assassin witch does the filtering do you just want your users to be able to pick the spam level they consider to be spam?
P.S. Contact groups are planed for Roundcube 0.4
Ok - well my understanding is this - currently on our mail server, they run spam assassin - I believe what it does is assign a ranking to a message and puts that in the header. It does not prevent users from receiving that mail. When they receive the mail, based on a filter they have set up, it will put mail into a spam folder - and yes, they configure what spam level goes into the spam folder.
I'm very open to another option, but we don't own our mailserver, our provider does, so I realize we are limited.