Not sure if you want to do this, but if you bought the brushes have AI on your computer you can convert the brushes over to AD. Unfortunately no fast process for it.
Select a brush and draw a straight line and copy the stroke or stroke pattern and create a PNG image of it in PS or gimp etc. with background black and stroke white.... Just leave a little extra black space on ends of image. Then go to AD and import the image you just saved.
On the Brushes panel, click the panel preferences icon and then select:
New Solid Brush—creates a basic, solid vector stroke.
New Textured Intensity Brush—creates a brush stroke based on the opacity values of a raster image. In the pop-up dialog, navigate to and select a file, and click Open.
New Textured Image Brush—creates a brush stroke based on the color values of a raster image. In the pop-up dialog, navigate to and select a file, and click Open.
The new brush is added to the selected category using default settings.
if you need the full details, here's how to do it.
https://www.designcuts.com/learning-hub/how-to-convert-adobe-illustrator-brushes-to-affinity-designer-brushes/
***also, if you are looking for Affinity Designer brushes. I have some here. https://creativemarket.com/Annex1
