Hi,
Have no experience with Beocenter 9000 but I have a Beomaster 7000 wich I have connected my Airport Express to its Tape2. Works very well and I think it would work as well for you.
You need a cable like this:
http://soundsheavenly.co.uk/bando-ipod.htm
Its a teleplug in one end wich you connect to your Aiport and in other side there is a DIN-5 connector wich you connect to Tape2. You might need a longer cable tho since maybe 1meter is not enough.
Cheers
Jesper