Grass spreads slowly to adjacent dirtblocks, so you just make a trial of it untill it reaches the point you want it to reach. Im told that the watering can speeds up this process. A tad later in the game you can use a silky jewel on a pickaxe/shovel so you can pick up the grassblocks without it turning into dirt.
I guess you want to know this 2: you can make dirt by putting organic material (apples, saplings, leaves, etc.) in a wooden barrel and wait. Yes you can automate this with a hopper
EDIT: you saved some time by saving the grass blocks, but you really need some way (silk touch i talked about) to store it, because the spreading method isnt very practical...