Dieter Schaefer , Mar 24, 2010; 01:52 p.m.
While you can take multiple exposures with your D90, there is no option to create an in-camera HDR with them. HDR involves creating a 32-bit HDR image followed by tone-mapping - no such capability in-camera with the D90.
Using multiple exposures too essentially lower overall contrast and dynamic range of a scene could be rather tricky in camera too - usually graduated ND filters are used for the purpose. You certainly have more flexibility by bracketing your shot and blending them later in post processing.