Photoshop brushes are raster. Illustrator brushes are vector so no drag and drop.
So you need a black on white brush outline as an image, bring it into illustrator, use live trace, and then make it into a brush.
Illustrator can make vectors out of raster image using live trace. When you select a raster image in Illustrator the live trace button should appear up the top. There are many different settings which can be found by holding live trace button to access 'Custom..' from the drop down menu. After Live trace you can use 'expand' and 'ungroup' to select individual vectors.
You can then make a new brush by selecting any vector in illustrator and in brushes tab choose 'new brush' There are a few different types to choose depending on how you want it to behave. Pattern Stamp etc.
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