To add some soft glow to the edge of lights, take your final render and just follow these steps. Let's start with this small area of the final render:
- Open it up and create a copy of the image layer in GIMP.
- Use Gaussian Blur of radius 5-10 depending on how big the image is.
- Now increase the brightness and contrast of this layer using Colors | Brightness. Contrast so you have something like this image:
- Set the layer to Hard Light or Lighten Only and adjust the Opacity to get the effect you want.
- Go to Layer | Merge Down to fix the ...