If your account includes support for checklists then you are given the option of creating a regular article or a checklist whenever you create a new article. This article will show you how to change the article type after you have created the article.
Support for changing the article type was added in version 4.0.9 of the desktop software.
Edit the article in the desktop editor
Open the article you want to edit in the desktop editor.
Change the Type in Settings
In the Article Inspector there is a Type menu with two options -
Select the type from the list and then save your changes.