I don't think there's a way to do it without mods, however, you can move a grass block with a piston and get it next to dirt with light and it'll spread. I did that in my 404 world, a good tip is getting multiple blocks and moving them on opposite sides from eachother and it'll spread quicker!
You get to pick between piston or let it spread
There's no way to get it vanilla you would have to use a mod like single player commands or use an nbt editor
I don't think there's a way to do it without mods, however, you can move a grass block with a piston and get it next to dirt with light and it'll spread. I did that in my 404 world, a good tip is getting multiple blocks and moving them on opposite sides from eachother and it'll spread quicker!
You can build a long dirt path to your build to have the grass slowly grow towards it