If you already have a job, it depends on your contract. For example, many agencies or consultancies have a line in their work contract that forbids you to do freelance work on the side. So first, check your contract if you have one to see what you legally can do.

Other than that, sure, you can do freelance work. When you are a junior professional having more work experience is always a good thing, be it freelance, volunteer or paid work. The challenge will be more on how to find freelance work. For that, your personal network is maybe one of the biggest assets you have.

Otherwise, you might have to look at platforms like Upwork, Fiverr or Toptal.