Bluetooth card i an Aluminium box?

Perhaps you could cut a "window" about the same size as the BT antenna pattern on the PCB, which is of course mounted in the box so that the antenna pattern shows through the window. Cover it up with a piece of plastic glued to the aluminum on the inside.

I dont know enough about BT RF design to add an external antenna when there isnt a connector for that already on the PCB...
 
The rectangular shaped silver pcb tracks near the battery connections on the pcb is the antenna (Meander Line Antenna), soldering a wire to it will detune the antenna and reduce the range of the RF signal.
There are only two solutions, either a plastic box so the RF signal can radiate without being shielded or a bluetooth board that has a pcb connector to fit an external antenna or a soldered coaxial fly lead with an antenna already fitted.