I am employed in a company but want to do a part-time job as a freelancer. Is it possible?

You can do a part-time freelance job, but in the majority of the free zones where you can get a freelance permit you need to provide a No Objection Certificate (NOC) from your visa sponsor (employer) in order to setup your freelance permit there.

Umm Al Quwain free zones provides permits without NOC.