How can I use Network Render on viewport's "Rendered" mode? I know that Blender can use Network Render to send jobs to several computers, but when working on a Cycles Renderer file I often switched to "Rendered" viewport shading to view the result as I move or spin around. 

While somewhat viewable, I wish I can utilize other computer's (probably with good GPU) resource to iterate through samples faster in the same way as Network Render. Can I do this?
No that isn't an available feature. The network render only supports rendering final images.

The best you can do to speedup viewport rendering is to use the machine with the best GPU that can be used by cycles.

Other options include using border render to only render the part of the scene you are most interested in.