I want to lower the ceiling in my bathroom and then add some IP44 Designer shower lights. I want to use the existing power supply and switch. How do I conect the new lights?

Judging by your use of the terminology "IP44" ( Ingress Protection rating), I will guess that your not in the USA, but in UK, Europe, or some other country. Really need to know your location to insure accurate advice.

Next, "How do I conect the new lights?" is too broad a question.

Can you tell more detail, what do you know about wiring, what type of wiring methods is existing, need to meet us halfway with some information.