There are some masks in the PSD, so it would be possible if you have some experience with an editing software that support PSD. You might need some manual tweaking depending on the result you're after.
Viewing post in 2D Hand Painted - Grassland Tileset comments
I have no experience on it, the reason I'm paying for sprites and tilemaps. If you were willing to do it I would be willing to pay. I have other ground tilemqps but I want it to match with what I bought from you.