If you place all your custom hatch in your custom folder, you can add support file search path. You should immediately see your custom hatch in pattern list. There is a comment that if you add user-defined hatch below that section, then when you upgrade your AutoCAD this file will be migrated correctly. Paste it at the bottom most of your acad.pat file. I use chainlink pattern from DotSoft website. Open another instance of Notepad and open your custom hatch. I suggest you make a backup of this file before you start to work. Find acad.pat and open it with text editors like notepad or notepad ++. It should be something like C:\Users\YOURWINDOWSACCOUNT\AppData\Roaming\Autodesk\AutoCAD XXXX – English\RXX.0\enu\Support.
Or you may just make your acad.pat corrupt. You may lose your custom pattern if you somehow need to reinstall AutoCAD or your Windows account corrupts. There are two ways to do this: you can combine your custom pattern with acad.pat or add a support file search path. So you have downloaded or created your own AutoCAD pattern.
