Stackexchange licences its content CC by attribution. As long as you clearly state that the content is from them, it shouldn't be a problem to re-use it. Many other sites do the same. (But you should check the licence for yourself before proceeding; don't just take my word for it!) However it always feels a bit cheap when a site copies content from somewhere else. It's always really obvious and never makes you look good. I would suggest avoiding doing this.
