-- function:
CREATE OR REPLACE FUNCTION number_or_zero
  (p_string IN VARCHAR2)
  RETURN       NUMBER
AS
BEGIN
  RETURN TO_NUMBER (p_string);
EXCEPTION
  WHEN VALUE_ERROR THEN
    RETURN 0;
END number_or_zero;
/

-- partial example control file:
LOAD DATA
INFILE delimited_flat.bcp
REPLACE INTO TABLE destination_table
FIELDS TERMINATED BY ','
TRAILING NULLCOLS
(numeric_field ("number_or_zero (:numeric_field)"))