My stance in these issues are quite liberal, in the "less than 6 arms is boring" sense.
It is quite correct that large morphological things are set in the womb, and what also many forget is that (most, including humans) are really not modular designs; while there ARE probably genes that you can just duplicate to make an extra set of arm, all the supporting structure like shoulderblades, nerves, etc. wouldn't work, and there would be a billion other problems to. There are a few things you could probably add without to much hassle thou, like tail and gills, as those actually exist at some point during embryological development but then are removed. Stuff like changing proportions or removing stuff that already exist should also not be to hard.
You can implant genes into an adult individual to some effect, one technique of doing this are those viruses mentioned, but remember this is not going to insert the gene into every cell, and in general probably have some other messy stuff associated with it. It's great for stuff like fixing insulin production and bone marrow, but not so much for this kind of "monster-making".
If you want stuff like wings (actually, that's still a bad example because humans are way to heavy, but maybe an extremely lightweight elite athlete could pull it of) the best bet is probably to use a variety of techniques to grow them separately in a lab on those scaffolding things they use for such stuff, and then attached surgically. This also has the advantage of being possible to do to an adult individual, making it voluntarily and removing a lot of those pesky moral objections, although it'd still probably be much easier to do if you added a few things before the womb to make such operations go easier, especially some kind of plasticity for nerves to form connections to the new bodyparts.