You don't have to do anything to APPX itself to cause it to create the "appx.stk" text file containing the stack trace. However, you do have to disable Dr. Watson. If you do not disable Dr. Watson, then an APPX error will invoke Dr. Watson rather than the APPX Stack Trace recorder.

To disable Dr. Watson, go into your NT system32 directory, and look for the file named drwtsn32.exe, and rename that file to drwtsn32.disabled. You might have to reboot after doing this, in order to get APPX to generate the stack trace. After you've gotten the stack trace info, you might want to rename drwtsn32.disabled back to drwtsn32.exe so that your next non-APPX error will be handled by it.