It maybe more convinient and cheaper? to buy a new set with built-in sub.
But OK, if you insist, u do it this way:
BT receiver -----(stereo)----->SUB-------->L+R.
Powered SUB must have the following features: Unpowered (RCA) jacks to L+R, Crossover knob, Gain knob (for sub only), must be able to take in stereo signal.
In the per-chance that your current L+R have a sub out jack, any sub then will do, for flexibility can take stereo or mono signal, may need a crossover, and a gain knob, if sub gain knob not present on L+R.