Some fancy mysql
You need to create the first block say category 1 with all its subcategories
Then you do a mysql export and in a text editor you need to change the main cat entry to another id value say whichever is the next autoincrement number next to come. You then need to edit the parent value in the export of all the subcats to be that new number. You then need to make the id value in all the subcats blank so it will follow the autoincrement on install.
Then take that tweaked mysql sql and run it.