Nope. There's no way to fill that gap that exists between the terrain and blocks.
But there's some alternatives you can take to bridge the gap. You can use the Plate block to bridge it as it's fairly thin, Or an even thinner option is the Iron Sheet block. But it may not look visually appealing to you...still, worth giving it a look as it'll give you a smooth surface to walk on when entering the door.
But there's some alternatives you can take to bridge the gap. You can use the Plate block to bridge it as it's fairly thin, Or an even thinner option is the Iron Sheet block. But it may not look visually appealing to you...still, worth giving it a look as it'll give you a smooth surface to walk on when entering the door.