How should I make scenes with atmospheric lighting brighter? I'm working with indoor lighting. I have a small room with two windows. I'm trying to create an atmospheric light. I have something like this

(sorry for a bad quality)
  

For that I used a spot lamp... The result is good but the room is too dark and it's hard to see objects in the room (if I place any). I thought about adding planes to the window frames, that would emit light, but how do I make plates let the atmospheric light through? Mostly the question is about where and how I can put additional lights, and whether I need to do that.

What I want is to have atmospheric light, but at the same time add some light to the room so that I can see other objects more clearly.
Whenever you are dealing with volume scatter and volume absorption your light sources will seem not as bright. Depending on the settings used for volume, a lot of the light is lost due to scattering, so it is normal to have lights that are much brighter than normal. 



Without changing your current setup or adding any more lights you can use the ASC-CDL node to change the values for your scene and control the contrast and brightness:



More info on the CDL see this post: What is the the ASC-CDL node?

For even greater and more nuanced control on your image, use the powerful  color management options, you and use custom Lookup Tables (LUTs) to control how the values for the scene are mapped into the sRGB values. 

More info here: Render with a wider dynamic range in cycles to produce photorealistic looking images