The information in this thread adds to the information in the support request I've been handling but I think I answered it in my last mail. MarkW is correct - The ABR file is the only file that can be imported when importing Photoshop brushes.

However, the Custom Brush tool is better suited for supporting custom brush heads if you have a PSD or other bitmap version of the head available. We're going to look at importing ABR files to the custom brush head collection as well to make this easier.