You will need:
A Windows PC (XP,7,8,10)
Adobe Photoshop (Optional)
Photoshop brush file (.abr)
Clip Studio (Manga Studio 5)
Download and install AbrMate (Free)
Click "Open Brush Set" and select the PS brush you want to convert.
Wait for the brushes to load into the program.
Click the "Export All Open Brushes to .pngs" and save them in a new folder.
Open Clip Studio.
File > Open > Select one of the .pngs you just created.
Open the Layers Window. (If you don't see it open it via Window > Layers)
In the upper left corner of the Layers Window, click the arrow icon with 3 little lines beside it.
Select "Convert Layer(H)..."
A "Convert Layer" box should pop up.
Change the "Expression color:" from Color to Gray. Click OK.