I previously worked for an NGO, during which I started to write 2 journal articles, which I completed during the first year of my PhD. I then wrote another one for the NGO a few months ago. I'd say as long as you have time (and as the above comments suggest, you can make time) then it's a good exercise to do so that you don't get too tired of your own subject, plus extra publications look good, especially outside the direct line of your research as it shows you're a well-rounded researcher. I'd definitely say go for it!