Doing a PhD in sociology as a computer scientist – is it possible? I am a PhD student in CS. I am also interested in the humanities and I would like to pursue a PhD in sociology. I know the traditional way is to repeat the cycle starting from undergrad studies but that would require a lot of time (which I do not have, given that I have other concerns).

Is this possible (ideally online)?
No, there's no legitimate way for you to get an online PhD in sociology (which is a social science, not humanities, but eh, lables are silly anyway), at least not with the profile you describe. You are also extremely unlikely to be considered for a normal sociology PhD programme. Speaking as a member of a sociology department, you quite simply do not have the background in the method and theory of the discipline needed to produce PhD level work. Just like I don't have the tools to suddenly do a CS PhD, you don't have the tools to do a sociology one. Successfully applying sociological theory and producing competent, critical analysis, not to mention data collection itself, is actually extremely difficult. You can't just casually pick it up. It recently took me 8 months and 40 pages of evidence just to get ethical approval to start a sociology project... 

What you can consider is a taught one year masters programme in sociology, either in the UK or elsewhere. Some of them are available online and some are willing to admit students without a sociology undergraduate degree if they can articulate why they are making a disciplinary shift. Mine wouldn't accept you with an undergrad in CS without a strong focus of that CS research being sociological in nature, but others might.