PIEDMONT FLT PI255 (A BOEING 737, N749N) WAS DSCNDG IN VISUAL CONDS ON AN IFR CLNC WHEN IT & A TWIN COMMANCHE (PIPER PA-30, N7399Y) CONVERGED & PASSED WITHIN A RPRTD 50 TO 100 FT OF EACH OTHER. THE NEAR COLLISION OCCURRED AS THE PA-30 WAS CRUISING WESTBOUND AT 8500 FT & FLT PI255 WAS DSCNDG TO 7000 FT. LANSING APCH CONTROL (CTL) HAD ADZD FLT PI255 OF VFR TRAFFIC (A BEECH E33) AT 6500 FT, BUT THE CTLR WAS NOT AWARE OF THE PA-30 SINCE ITS RADAR RETURN WAS BLOCKED OUT BY AN OVERLAP WITH THE BEECH E33, 2000 FT BELOW THE PA-30. THE PLT OF THE BEECH E33 WAS BEING PROVIDED VFR ADVISORIES BY APCH CTL. THE PA-30 WAS NOT ON A FLT PLAN & WAS NOT IN RADIO CONTACT WITH ATC. NEITHER OF THE CREWS INVOLVED IN THE NEARCOLLISION SAW THE OTHER ACFT. ALL 3 ACFT WERE TRANSPONDER EQUIPPED WITH ALT RPRTG CAPABILITY & ALL WERE SQUAWKING EITHERAN ASSIGNED OR AN APPROPRIATE CODE SUPPLEMENTED BY ALT REPLY.