Yes you can! One of the many advantages of digital post production is being able to change the background or place the subject into a new background. Many times I go into a client's premises and we don't have room to get 'the shot' or the client is so terribly busy they only have time to stand in front of a white background in a small office.
This photo was taken on a white background and I dropped him onto this dark vignetted background. Let me know your comments?