I think that this is usually a bad idea.  It can mess up filters, graphs,
pivottables, ...

If you want to make it look double spaced, then increase the rowheight.

But you can do it a few ways.

One way (manual):
Insert a new helper column
put =row() in the top cell of that new column and drag down (>400 rows)
Then convert those formulas to values (edit|copy, edit|paste special|values)

Then copy those numbers to the rows directly beneath the >400 rows.

Now select the whole range (row 1 to > row 800).

Sort your data by that column.
Delete the helper column.

Another way is to use a macro.

This picks out a column that has data in it on the last row -- I used column A.

Option Explicit
Sub testme()
   Dim LastRow As Long
   Dim FirstRow As Long
   Dim iRow As Long
   
   With ActiveSheet
       FirstRow = 2
       LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
       
       For iRow = LastRow To FirstRow Step -1
           'to increase the rowheight to twice the height
           '.Rows(iRow).RowHeight = 2 * .Rows(iRow).RowHeight
           'to insert a new row
           .Rows(iRow).Insert
       Next iRow
   End With            
End Sub

If you're new to macros, you may want to read David McRitchie's intro at:
http://www.mvps.org/dmcritchie/excel/getstarted.htm