That's possible. But we are not there jet. I think they have a new method called, CRISPR. With this method it's possible. But the human DNA and it's makeup is so complicated, so we are not able to do much jet. The problem is that if we change something in our DNA (genetic code) there is so much change that comes with that tiny change, so we don't know what's going to happen.
Let us say:
Maybe you can get a big dick with this method, but it results on some really negative side affect on something else. Maybe your neurons starts to decay, or you can only live to your 20's before your body destroys itself. These are only examples that's not connect to the reality. But it's practically this dilemma you are dealing with. Out body is so complicated.