Insert a field at a specific position
I know of no means to do what you want using the table design dialog in OpenOffice/LibreOffice. The method below has been recommended on several forums for re-ordering fields in a table using the embedded database. If your table is referenced by constraints or views, then they must be deleted and rebuilt along with any relationships.
Copy the table, then choose Paste Special and choose Data source table as the type to paste. Choose to copy Definitions and data. In the Apply Columns dialog that follows, transfer the existing columns to the new table in the order you wish. After the table is created to your satisfaction, delete your original table and rename the new table with the original name.