Hi Martin,

Here you can find online documentation about Hatch pattern customization:
AutoCAD 2018 for Mac Help: About Custom Hatch Patterns and Hatch Pattern Definitions
http://help-beta.autodesk.com/view/ACDMAC/2018/ENU/?guid=GUID-A6F2E6FF-1717-44B6-A476-0CA817ADD77E

And here:
AutoCAD 2018 for Mac Help: To Create a Simple Hatch Pattern
http://help-beta.autodesk.com/view/ACDMAC/2018/ENU/?guid=GUID-7E1BFC62-C5F2-48C6-84D7-71FAD4DF0BEF
how to edit (create them)

So, on Mac you can use TextEdit.app to edit (create) .PAT files.

Maxim