You could hook it up with a din to din lead i.e from Aux on the overture to take in on the BM5500 - but you would get problems with using the remote as both would respond to Radio etc so I would imagine you would connect aux on the Overture to CD on the BM5500 and use the beomaster radio and the overture as a CD player - the other way if you are just using the BM as an amp may be to get Powerlink to Din Lead - I'm not sure as I've not done it but seem to recall that there are Pre-Amp to Power Amp sockets on the Beomaster - so in theory you could take the Powerlink Line out from the Overture to the Power Amp in Sockets on the Beomaster and use it that way but you would lose any other functionality in the beomaster
The other option would be to replace the 5500 with a Beolink amp - they are cheap on ebay (upto £150) and this would drive your speakers from the powerlink sockets on the Overture. Or more expensive to purchase beolab speakers.
Beovision Avant 32 RF, DVD1, Beovision 1, MX4002, Beound 3000, Beolab Penta MKII, Beovox Penta, Beolit 707, Beolink Passive, Beovox C30, Beocom 4, Beogram TX, 4 x Beo4, Form 1 & 2, Beocenter 7700, Beovox S65,