You may need to do a software upgrade to add wireless surround speakers https/www.sony.com/electronics/support/res/manuals/4589/45898930M.pdf
The speakers that Sony sells to use with that soundbar https/www.sony.com/electronics/wireless-surround-sound
They mention something called SongPal Link so you might have to use only those speakers.
If you have a BT speaker try and pair it with the soundbar. Worse case it won't work. Best case you don't need to buy specific speakers. Two mono BT speakers might not give you left and right surround. You would have to pair each one separately and designate which is left or right.