Change the dna of a living organism, then get it to express changes?

Just a little thought experiment. But lets say you wanted to go about changing the dna of a living organism and getting it to actually express this change without actually killing it. And i'm not talking about something like changing the color of its skin or eye color. I mean really messing the thing up (new organs, new limbs, increasing size of body parts). In short making it do things that its god given genes would never let it do. I've heard of making gene modifications in embryos and gamete cells and then having the changes expressed when the organism is born. I guess a good example of this would be those zebra fish they made fluorescent (a href="http://en.wikipedia.org/wiki/Glofish#Early_development)." rel="nofollow"http://en.wikipedia.org/wiki/Glofish#Ear…/a I can see how its possible to actually change the dna of a mature organism (microinjection, plasmids, that sort of thing) but how can they be made to express these traits (proteins, stem cells, cancers?). So are there any ideas on how could you get zebra fish to glow or hamsters to grow gills while they are still alive without actually killing the s?