Here is a program that can be used to edit the XML files used to store local codes for the Nintendo DS Action Replay. It can also add new games, codes, or folders to existing files.

Warning: This program is free software. It is has been created WITHOUT permission of anyone behind the DS Action Replay. USE AT YOUR OWN RISK! There is NO support for this FREE program. The author takes no responsibility for problems caused by this program. If your computer starts on fire, it's your fault because you probably poured something on it.

Download - Note: This program requires the Java runtime, you can get it from http://www.java.com/ if you don't already have it installed.
Additionally, an EXE version has been created for Windows users having problems running the .JAR version. It can be downloaded here. Java is still required for the EXE version.
See the readme file for details about changes made in each release.