In SSIS, to replace a null value - used the Derived Column box with the following systax:

ISNULL( [FLDA]) == TRUE ? 'X' : [FLDA]

Relates to ' If FLDA is null, move 'X' to FLDA, else move FLDA to FLDA'.

This will check FLDA for nulls - it it is equal to nulls, it will replace it with 'X' (or any other character you specify), otherwise, it just keeps the content of FLDA intact.