Not sure of what you exactly ...

If you want to combine tests, you can write this :

Rule “test filter”
When
      $cm: ConditionMatrix()
      $document:   ArrayList() from collect ( Document (user.isPrivateName == $cm.isPrivateName, Payment.isLegal == $cm.isLegal) )
Then
                //do something
End

If you want to make the filter more dynamic and write only one "collect" rule, I don't see any out-of-the-box method.
You can add a method in ConditionMatrix that accepts a Document and write this rule :
Rule “test filter”
When
      $cm: ConditionMatrix()
      $document:   ArrayList() from collect ( Document (eval($cm.accept(this)) ) )
Then
                //do something
End

Of course you have to implement the "accept" method in Java (and you can then do several sub-classes for you filters)