Registering a replacement Binatone 1820 handset?

I have a Binatone 1820 answering machine/triple handset cordless phone system. One of the handsets got damaged and as I was unable to buy a replacement handset on its own I bought a single system with answering machine base, which the seller assured me I could register to my existing system. But as the new handset is registered to its own base how can I register it to my existing triple system.