At the back of your BM6500 there should be a lot 7 pin DIN plugs named, CD, PHONO (which are inputs), A.TAPE, A.TAPE2, TV/AUX (which are in- and outputs. There are also two Powerlink outputs ( 8 pin DIN), which are preamp outs for active speakers (Beolabs).
I`d take A.TAPE2 port for your adapter. You can activate this port pressing A.TAPE2 on your BL7000, or SHIFT - A.TAPE on BL5000/1000, or N.MUSIC on newer Beo4s. Maybe A.TAPE2 will not be displayed on your BL7000. You can add this button.
Stefan