How do I get a modified shape plane into the content browser?

I’ve applied a grass alpha image to a shape plane from the content library to make grass. I’m having a problem getting that modified mesh into the foliage mode so I can paint instances of it on my landscape. I’ve exported it as an fbx but on reimport it is invisible. How do I turn it into a mesh that can drag and drop into the add foliage type box?

Thanks