5.1[6 channel] To 7.1[8channel] Analogue Audio

i have a 5.1 reciver and am looking at either the sony s550 or pana db55 wich both have 7.1 analogue outs.. but i dont know what i would do with the two extra channels? i was thinking maybe use y connecters on the surround L/R inputs of the reciever and then i would be able to connect both surround L/R and rear surround L/R of the BD player into the surround L/R (now with a Y connectors) input of the 5.1 reciever, confused yet ?? but i thought this is the wrong way to do it. as i can't get the manual for either moedel i dont know if you can simply set output to 5.1 onthe analogues, but if you cant what can i do ( appart from simply not plug the rear surround L/R in ). Or better is there any converter box to plug 7.1 channels into one end and have 5.1 channels out the other ( evan if this is a bad idea i would love to see pics if these are avaliable ). Hope to hear from you guys soon. Regards Marty.