"Home Theater" boxes are known around here as HTIB and they are extremely inflexible and you have this wish that somehow it can aid you toward attaching a modern blue ray player to an old TV? Highly unlikely.
First off HTIB boxes can decode surround from its built-in DVD and that's it, it cannot do surround from anything else, not your BR player, not your TV. If you wish to listen to BlueRay surround you must upgrade this HTIB to something else, here we suggest an AV Receiver for maximum flexibility and future expansion, but certainly you can buy another HTIB with BlueRay player, but once again you are warned it can't do anything else.
If you don't mind listening to stereo only, BlueRay player with stereo audio out to HTIB AUX-IN, if it has one, and BluRay (Component hookup) ----> TV (again if TV has it). If TV has no Component (a 5 cables connection) then you are looking for HDMI to S-Video/Composite converter and the image will look subpar.