From WADder Wiki

Usage

You can use this tool to create .mym files from themes. However, these .mym will only work on the System Menu version the original theme worked on (those they can be used on any region!). Here is how to do it:

• Install the Converter.
• Run it, and read the usage dialog.
• When it asks for the version, press Yes for 4.0 or No for 3.2.
• Then, select the theme you want to convert.
• It will run for around 5 seconds.
• When you are asked to save the file, browse anywhere and save as any file name, but add .mym to the end of the file name.
• See Installing Themes from MyScripts to install.

Finding The Converter

Since SoftMii.org went offline, it has become difficult to find the tool. It is packed in the WADder Portapack, and a link is buried somewhere on the forums.