Hello,
HaxeFlixel uses the
MIT License. As the website succinctly puts it, MIT is a license that "lets people do anything with your code with proper attribution and without warranty".
As I understand it, you should be fine to do anything as long as you include the original license file(s) of any and all dependencies along with your own.
An actual member of the HaxeFlixel dev team will probably chime in with his or her take. Though if you really want peace of mind, talking to a legal professional is always a possibility.
Domagoj