You will want the service manual for the subwoofer (well more likely you will get a service manual for the whole system). Then you have 2 sort of ways you might be able to go.
Patch an input into the existing control board (hence the service manual for the wiring diagram)
OR
Replace the control board with one that matches the the speaker and has the inputs you need. (hence the service manual for the exact speaker used)
After the annoying parts then you just need to make a hole to put the jack in and you now have a wired sub where you once had a wireless one.
Hopefully someone has already done this before you and found all the required parts and pitfalls and made a guide, and with a popular brand like Philips there is a chance.
Edit: also
This is not always the case.