There are a few converters out there, but the one that I use myself and recommend is @Patrick's map converter. It supports both ways. https://mtaclub.eu/converters
Exporting from Max requires Kam's map IO or Goldfish's emap IO.
If you are experiencing accuracy issues with the editor, there may be resources out there that you can use if you rather map in the editor. https://community.multitheftauto.com/