ALTER TABLE statement limitations
When the dBASE or Paradox driver is used, once an index is created and a new record added, the structure of the table can't be changed by the ALTER TABLE
statement unless the index is dropped and the contents of the table are deleted.
ALTER TABLE
statements aren't supported for the Microsoft Excel or Text drivers.
Note
When you use the Paradox driver without implementing the Borland Database Engine, ALTER TABLE
statements aren't supported; only read and append statements are allowed.