You CAN have an FAA IR added to a 61.75 certificate which then gives you full instrument privileges in an 'N' reg aircraft for as long as you meet the FAA instrument currency requirements AND keep the foreign licence on which your 61.75 certificate is based, current.
But just to be clear adding an FAA IR to a 61.75 adds nothing to the underlying JAA Licence.
.. although - please correct me if I am wrong - but with an FAA IR added on to an 61.75 FAA PPL you can fly at night (as well as in IMC).
No, not unless you have a night qualification on the underlying licence.
My friend is in the US doing an FAA IR at the moment and just hit the hurdle of the night qualification. He has a JAA PPL with no night qualification and a 61.75 certificate that he was going to add the IR to while he built hours for an FAA CPL. As he did not have a JAA night qualification he could not do the IR and had to do an FAA PPL first which he did by doing the required night hours and cross country.