This softball drill is basically to get the girls focused on stopping the ball when it's coming to them. In this softball drill, eliminate the throw entirely so that their only focus is stopping that ball. Make sure they are approaching the ball well. Make sure they are using proper fielding technique and footwork. Emphasize these things and most of all, emphasize stopping the ball!
You can't do anything on defense unless they control the ball.
Since the players won't be making a throw, they can concentrate fully on fielding technique. Then simply have them run the ball over to a bucket and run back in line to field again.
Of course, where you place the bucket is up to you. If you only have a few players in the drill you'll probably want to keep it close so the flow of the drill isn't interrupted by the players having to run over and drop the ball off. If you have more players you can move the bucket further away to keep the players moving and make the drill physically challenging.