I’m a Monsanto scientist who has more than 20 years of experience with genetic modification of plants. I will try to answer your question, even though I don’t ever do experiments on animals, certainly not on humans, of course!
Can humans be genetically modified…but a much bigger question is should humans be genetically modified?
There are two ways to think about genetic modification of humans (or any animal). One way is modification of somatic cells, and the other is the modification of germline cells. Your question is probably related to germline modifications, but first let me talk about the difference between the two, because genetic modification of human somatic cells has been done for decades. Genetic modification of germline cells to make a genetically modified human has not.
The modification of human somatic cells, also known as somatic cell gene therapy, is a procedure in which cells, for example bone marrow or blood cells, are removed from a patient, genetically modified in a lab to try to “repair” or “replace” a defective gene that causes a disease, and then transferred back into the patient. Sometimes this involves adding a correct copy of the defective gene. In the future, this will increasingly involve the use of a new method, called gene editing, to correct the defective copy itself. The first attempts to do somatic cell gene therapy occurred in the 1980s, and continue to the present day. Thousands of clinical trials have happened and continue to happen. In patients of somatic cell gene therapy, some of the cells in the human body are genetically modified, but typically a small fraction.
Your question probably pertains to genetic modification of germline cells, which would result in a human in which every cell in the body, and all of the children, grandchildren, great-grandchildren, etc., of that person, would be genetically modified. To date this has not happened anywhere in the world, so it is not possible to say with certainty that it can be done, but it is quite common in many other species of mammals, including monkeys, especially with recent advances in the area of CRISPR mediated gene editing. So, although it hasn’t been done yet, it seems likely that it can be done in humans. There have been reports of genetic modification of human embryonic cells in China and in the U.S., but all of those cases the experiments did not proceed to the point of a human embryo or baby.
Even if germline genetic modification becomes a reality in humans, it is one thing to use it to repair a mutation that causes a disease, but it is quite another to use it to “enhance” human traits.
The latter captures people’s imagination, but to do so would require a level of knowledge about complex genetic traits that just doesn’t exist today, so even if the laboratory techniques exist, the data science required to do so doesn’t.
As you can imagine, there are many ongoing ethical, moral, and legal debates about whether or not germline genetic modification of humans should occur. A colleague of mine wrote about this, in the context of gene editing, in a recent LinkedIn article. Many countries and major scientific organizations have banned research, for now, that would lead to a genetically modified human, but there is also growing interest in the potential for germline genetic modification, especially gene editing, for treatments of devastating, heritable diseases for which there are few, if any other options.