The award calendar is showing availability for virtually every day in September for flights from NYC to AMS. And KLM is usually easy enough to get the intra-European legs with.

VS often has plenty of availability out of NYC to London, but this may be more of a hassle than going through AMS.

You will just have to force the agent to look. When they say, we only have EasyPass, say:

"Can you get me to Amsterdam? Or Paris? Or London?"

Then say:

"Are there any connecting flights from there on KLM? Or Air France? Or Alitalia?

If you need/want to fly on a non-stop, then you might be out of luck, but I would think you could get there via some routing.