Is it possible to hook up an external mic to a Nikon D90 to get better sound for video?
Nope, it sure doesn't.  On that camera, its only got an internal mic.  Your options are:


Record sound otherwise and then sync up in post.
Buy a new camera like a D7000 that will take an external mic.
Use an actual video camera that takes an external mic.
Possibly tape some material to mimic a windscreen over the existing D90 mic, to help a little in some situations.
Live with the limitation.