Hello Yes very possible and even moreso if you book "cruise only" then the agents or you can book the flights separately....Heather.
As we have no idea what cruise you are talking about it is hard to advise. Of course you can book your own flights from Amsterdam to whereever you want to go to. I doubt if any cruise line will book a flight from AMS thought.
There is nothing to stop you booking a "cruise only" and then booking your air travel from wherever you want separately. However this will probably work out more expensive than taking the dedicated "cruise" flight. There are cruise connecting flights direct from Cardiff - P&O for one have them to the Caribbean. Bria.