It's been possible for a long time now. Take a look at this article related to trials with retroviral vectors used for gene therapy trials. A single child developed leukemia, and following that trials across the world were suspended.
The article although notes that many countries decided to proceed with trials, since as they say "for the greater good". But mainstream media articles tend to glorify the actual reasons behind such decisions, because such questions are governed by much more complex ethical considerations.
But moving on this recognised the need for targeted gene therapies, so one of the most promising techniques right now is the Crispr/Cas9 technique as noted by the comments above above.
So do we have the technology? Yes we do.
Should we go forward with it?
I would say no. But, I would also say yes.
Why such a division in opinion?
There is much that we do not understand. Until a few years ago, the non-coding part of the genome was just junk. Now, it's an essential regulatory partner. Even more recently, the understanding of chromosome architecture was not understood, now we know that Chromosome architecture has a role to play in gene expression. In 2002 epigenomics was in it's infancy, we believed "central dogma" to be a dogma, non-coding RNAs were just some junk that you tended to pick up because the transcription machinery did not know what it was doing, and chromosome architecture? that hardly mattered in the grander scheme of the cell. All in all the cell was a much simpler place. Now we know more than we did yesterday and less than we will know tomorrow, so if I was a person whose entire life is a prolonged episode of suffering I would take my current options. If I was a person who could live with a mild discomfort I would work around it.