My step dad is buried here and my Mom has since passed away and wanted some of her ashes spread on his grave. Is it possible to do that?

My Father is buried there and my Mother's ashes are with him in the same plot. His inscription is on one side of the headstone and her's is on the other. You can do the same for them.

She would be buried with him same plot

You will need to call them. Why isn't your mom being buried right next to your shed dad ? It's her right.