Absolutely. You can take the CareerExplorer career test even if you already have a job. In fact, CareerExplorer takes into account your past careers, and uses this information to shape your recommendations. Additionally, because you have working experience, you may have a better grasp of what things you like or dislike in the work environment, making you better suited to take a career test.
People typically find jobs casually and through acquaintances, ending up with a career they may not fully enjoy. CareerExplorer helps to break that mold by offering a scientifically valid way of finding your ideal career. So regardless if you're in your first job, or in the middle of a career, being educated about your options is always a good thing.