Continuing the discussion from Spotty brushes:
The short answer is no. MyPaint cannot use Photoshop brushes. Long anwer is that MyPaint and Photoshop fundamentally use two different methods to rendering a brush. Most, but not all ABR(Photoshop) brush files uses a black and white image file that can act as the “brush” base and then the program mathmatically manipulates the size, opacity, ect of that brush. MyPaint on the other hand actually builds a brush from a single dab(dot) and manipulates that dab according the perimeters you set in the brush settings. So fundamentally, MyPaint can’t use ABR brush files like Photoshop uses them.
Now there is a work around for this. MyPaint dose have a feature that allows any layer to be open in another program. So you can in essence import the ABR brush file into GIMP for example. Then you can use MyPaint’s open external application feature and use the brush in GIMP. Once done, save the layer with GIMP’s export feature and the changes should propagate on MyPaint.
Here is some more information about using GIMP with MyPaint layer feature: Using Edit with External App Tool with GIMP