I am trying to do the exact same thing.
I have a nokia 6310i, with car kit installed in my audi.
I have just got a Orange SPV M1000 and have been trying to pair the 6310i and the xda2 together, with the idea of using the 6310i + car kit to use the existing kit.
I have paired them, but straight away disconnects at the 6310i.
Does any have any ideas why this can't be done.