You can get a BT retrofit kit, but it wont be anywhere as intuitive as the new e90's RAD2(head unit) units are.
As far as needing to replace the diversity antenna module when using a new headunit: There is a much cheaper alternative.
Simply connect the connector(Smaller round one) to a 12v source that is attached to KL15, such as one of the radio pins.
This is all the module needs to achieve the same reception as it had with an older head unit that had both of the plugs.
It draws very little current, so no worries about overloading a circuit.