You would have to to place a box light that covers everything in the level and keep it at a somewhat low number for light value (like 0.2 for dark, 0.4 for mid, and 0.7 for bright, and 1.0 for max). Then use this to change the value of it:
FadeLightTo(string& asLightName, float afR, float afG, float afB, float afA, float afRadius, float afTime);
Changes the properties of a light.
asLightName - internal name
afR - red value
afG - green value
afB - blue value
afA - alpha value
afRadius - radius of the light. -1 means keeping the radius
afTime - time in seconds until change is done
Then you could use it to change back to the original lighting etc.