Hay! So, I can't say much about whether it will be a waste of time, but I can say that you might not even need to do it. For context, my goal is to complete my MSc in forensic psychology and eventually try for my PhD. Also, I'm an international student. I was told that the only way I'd be able to get into a master's program is by completing a conversion course first. However, I recently got accepted into a master's program without the conversion. Although it's true that I might need the conversion course in the future (if my undergrad isn't accredited (which I'm still waiting to find out)), it at least means I won't possibly be wasting the money now.
Also, as far as I'm aware, to proceed to doctorate level, you need to complete an MSc, not just an MSc conversion. I could be wrong though...