there are many ways to do this.. but it's hard to give you exact advice without a lot more information.
so is this just for an item collect? like in minecraft when you mine a block? you want the block to come up?
one easy way is to make an image point on the player sprite. Put the image point below the character. then when digging down you can have the Player object spawn a new object at image point(1).
Or just reference a position below the player. System Create Object block at player.x, player.y + 200 (so it's below the player)