Prepaid Contract in the US With a European Phone


I'm living for four months in the US and need a prepaid phone contract. Are there any options where I can get a prepaid contract without a phone? I got a Lumia recently and do not want to buy a new smartphone. I think the European version of the Lumia works only on AT&T's network... so do you know about any MVNOs that have prepaid plans that do not require you to buy a (throw away) phone?

Thanks for your help :-)