Hatch files with the ".pat" extension must be placed in the "support" folder to be recognized and used by Autocad.
The "support" folder is located in:
documents and settings -> username -> application data -> Autodesk -> AutoCAD .... always go forward until you find the support folder.
To view all folders properly, you must enable the option to view hidden files and folders (in windows)
The complete path to locate the support folder exactly can be viewed in autocad> menu> tools> Options> File> "Support file search path".
Once you have found the "support" folder, keep the folder open.
Select and copy all the .pat files that you intend to import into autocad and paste them all into the support folder.
Close everything, the hatch pattern files have been inserted into autocad.
From now on when you have to insert a hatch in the "palette of hatch patterns" as well as the predefined ANSI, ISO and Other templates we will find in Custom, the list of new hatch patterns that we have just imported.
Choose the pattern via the preview on the right and with the ok the hatch pattern is chosen.
See the hatch patterns available on Archweb