I looked at doing this to my pair of BL3's, i had to buy two Powerlink to male RCA (part code 6270856), one for each speaker which then connected into a cable US sound adaptor cable (part code 6270433), which basically has two female RCA cables on one end and the headphone end that goes into the iPod or similar MP3 player.
In the end i bought a 3200 and then bought a Powerlink to headphone jack off ebay which connected straight into the 3200, a much better solution however if i could do it over again i would have bought the BS4.
If you're in the UK i have the sound adaptor cable however only one of the powerlink to male rca, which i'd be happy to sell, so you'd need to get another one powerlink to male RCA.
Beosystem 3, Beosound 4, Beolab 9, Beolab 3, Beolab 8000