This functionality isn't built into the WinGrid control and you should log a new product idea on the NetAdvantage for Windows Forms product idea page for this if you are looking for this functionality.
You may be able to do something like this using a creation filter to move the existing rows if there are not enough rows to fill the grid. If you do pursue this you would still need to sort the rows so that they are already in the correct order first and they you would move the existing rows down when they don't fill the entire grid. There are more details on creation filters in the help.