I am going to highly recommend against this.
an mpg file is a compressed file, to uncompress (Which you dont get any of the original information back that you lost when compressing), then edit, the recompress is a very bad idea.
You will suffer some serious loss in quality by doing this. This was one the first things I ever tried when I started doing video and learned, very quickly, that it does not work well.
It can be done, but as a good friend of mine says "Just because you can, doesn't mean you should"