Is it Wireless or Bluetooth?, if its Bluetooth then, as said, its fairly easy to get an Line Out or Headphone adapter to do the job, if its Wireless then could prove more challenging as the Wireless protocol is probably more suited to the Manufacturers Transmitter spec.
Personally I wouldn't bother with the above, just take the Speakers apart and fit a switch and some external connectors that are wired directly to the Speaker, just make sure you get on the right side of any Crossovers fitted, there is a slim chance, but unlikely, that the speakers may be internally Active Driven per Speaker or offer Sub Base output, if thats the case then you need to to apply a bit more work.
i actually did this to a set of Loewe Wireless Floor Standing TV Speakers (made by Linn), I bought them for £1 at a closing down TV/HiFi Shop as they had Sold the TV off before hand (TV had the Wireless Speaker Transmitter built in), by passed the Wireless gubbins and wired them directly to my own Loewe TV Speaker Out, sounded brilliant.