That's a good suggestion, I think I going to write an article about that and share it later here.

Generally, the key here actually is to take two different photos with the same composition but different settings.

Let's take an example:

Photo A (for foreground) - longer shutter speed for a bright foreground.

Without moving your camera, you capture another shoot.

Photo B (selfie) - shorter shutter speed (maybe around 15seconds) and boosting up the ISO.

Then just blend the person from photo B into photo A, DA!DA! Done! ^^
Of course, you can choose to use Photo B alone instead of blending, but then will have a result with much more noises.

Hope my explanation help.