Hi

I currently for a company and I am paid a monthly salary. They outsource a certain job to independent businesses and I want to try and see if I can do this job once off to make some extra money. I want to know the following..
1. To be paid I would need to invoice them, can I invoice using my own name or do I need to make up a company name? if I need to make up a company name, am I allowed to invoice for a once off job without registering the company? You could say this is a once off free-lance job but it is complicated because I am already employed by the company I am freelancing for.
2. Would I need to declare this to SARS? I do currently earn enough to pay tax.

This is my first time posting on here, I hope this makes sense!
Thanks in advance