I recently did a Photoshop project using some ABR files. The project turned out fine, but the person wanted to use some of the brushes outside of Photoshop. Thus my hunt began for a way to convert ABR files. You can convert them to PNG files using a freeware utility called ABR Viewer 2.0. Once they are PNG files, you can use them in almost any image program. If you'd like a detailed tutorial on how to convert the ABR files to Photopaint Nibs. There's an excellent tut available HERE.
Enjoy!