What's keeping you at SME 5.5 anyhow? That's a REALLY old version of SME. You should really consider upgrading to something more current.
In any event, I believe you maybe be able to process email through spamassassin on the SME 6 box, then pass off processing to the 5.5 box using the reverse of what I told you to do earlier. Setup your SME 6 box to function under your primary domain, then set the Deligate Mail Server option in this box to point to the 5.5 box.
Greg