If the wired speaker is passive you would connect it in parallel with the speaker inside the cabinet. The BT speaker amp is not likely to have enough power for both speakers and it could damage the BT speaker amp.
If the wired speaker is active then you have to find the output of the BT receiver and connect the wired speaker there. If the BT receiver is a separate board then it should not be too hard. If there is only one circuit board then you will need more details and more knowledge,