Three or More Task Substep Levels

Folks,

I am converting some unstructured to structured content to produce some simple introductory DITA XML files to port to Eclipse Help.  Most of the content is text, procedures, and images (originally in both text and procedures).  The procedures aren't used in other content, so I started out with a DITA concept XML file and put in task elements.  Here are my issues:

Out-of-the-box, task lists seem to be limited to 2 levels (1. and 1.a.).  Is it possible to create at least a third level (1.a.i.)?  If so, how is this done?

Thanks