You can unlock the EXCHAPTERs under EXTRA at the main menu by meeting the following conditions.

Chapter	How to unlock
EXCHAP 1	Get the Good Ending in Chapter 1.
EXCHAP 10	Get the 2nd Ending in Chapter 5.
EXCHAP 2	Get the Good Ending in Chapter 3.
EXCHAP 3	Get the 6th Ending in Chapter 4.
EXCHAP 4	Get the Good Ending in Chapter 4.
EXCHAP 5	Get the 4th Ending in Chapter 5.
EXCHAP 6	Get the Good Ending in Chapter 5.
EXCHAP 7	Get the 8th ending in Chapter 5.
EXCHAP 8	Get the 6th Ending in Chapter 5.
EXCHAP 9	Get the Ending in EXCHAP6.