One way is to extend the original classes and add the desired attributes, then insert the extended class into WorkingMemory. You could then use RHS to modify matched facts. Another way is to inject "related" facts. So your LHS matches a fact and you insert a new "related" fact in the RHS. Rules that must evaluate data from both facts can the do something like this:

when
    $b : BaseFact($key : id)
    $r : RelatedFact(parentKey == $key)
then
    ...

I have used both albeit on small sets of facts and both worked fine.

Earnie!