Is it possible for a junior service designer to do freelance work?
Yes. The challenge will be to find work. Platforms like Upwork, Fiverr and Toptal can help for that if you are not a great networker.
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.
Related articles
What are the mental health challenges a service designer will be faced with?
In short: learn to deal with negative feedback, give enough time for life and learning, keep track of the good stuff, and don't forget to sleep.
Can a service designer survive a recession?
In short: Yes, if you can also provide a lean and low-hanging fruit approach that is focused on the present. No, if you are "only" focused on long-term strategic work.
What are tips to become a senior service designer?
In short: mentor others, teach others, try out new technics.