You cannot do this from with Settings of the default camera.
You could look at another camera app like:
Camera Streak : Blog:Camera Streak (http://www.addictivetips.com/mobile/camera-streak-for-android-simulates-long-exposure-photography/Blog).
Or you could roll your own, here is the relevant details you need to change:
http://developer.android.com/reference/android/hardware/Camera.Parameters.html