This is totally sci-fi fiction but potentially possible. A provirus is a virus whose DNA actually gets added into the host DNA.
If you could create a virus without the ability to replicate it's self, and if you could choose the DNA code it carries you would have a virus that is harmless (so long as the dna code in it is harmless).
it would be a way of transporting dna to your body. and if this virus infected most the cells in the body you could add or change features of that body.
for instance you could cure genetic diseases by replacing bad genes. you could change features such as eye color or skin tone.
every cell in our body is replaced every 7 years (except perhaps nerve cells I don't know). therefore in theory you could change your own genome within a 7 year cycle.
for more information on proviruses see http://www.khanacademy.org/video/viruses http://en.wikipedia.org/wiki/Provirus