is it possible get a JAA Multiengine and instrumental ratings in USA
You can get a JAA MEP rating but you can't get an Instrument Rating in the USA, that will need to be carried out in JAA land. You could however add a IMC to your JAA PPL in the states or if you're hell bend on getting a IR in the US then you could add a FAA IR to a converted JAA > FAA PPL.
Good luck.