How to get SQL for DDL commands in Sql developer?
Oracle knew you wanted this feature. I'm using SQL Developer 4.0.0.13.80 and it works like you want it to.  You were probably using an older version when you posted your question.  I go to Edit Table, add a column in the GUI, and then pick the item called DDL in the dialog.  There is a radio button to show either the full CREATE statement DDL, or just the DDL update for the current edit.