Do it in camera? You mean the camera itself can't only shoot a load of pictures with different settings automagically but also combine the photos into one picture?
Excactly. You will find that function in the filebrowser. Just mark several CRW files by pressing "left" (At least on my cam), then open up the menu by pressing "right", and select "Raw sum" or "Raw average" - whatever you like. Raw sum will simply add every single pixle, and therefore increase the exposure, whereas "Raw average" will calculate the average value of all pixles. Raw average is a fine feature to remove noise in static scenes.
The calculation of the new RAW will take some time, and when its done you finde file with a *.WAV extension. Select that, open up the menu by pressing "left" again, then select "Raw Develop". You'll see a popup window saying "Please switch camera to record mode and take one shot." - do that. The camera will develop the selected RAW file instead of taking a new shot.