It is possible to do, yes. However, (this coming from trying to do the exact same thing), windows 7 doesn't play nicely with the GPT formatting that windows 8 requires, meaning you're going to have issues trying to do a dual-boot.
You could replace windows 8 with 7 easily, or install 7 to a different hard drive.
There are guides online on how to dual-boot win 8 and 7 from the same drive - starting with a blank drive, you install windows 7 and then 8, but there are a few tricks to it.