Have a look at these user-exits:
IQSM0001 Automatic serial number assignment
IQSM0002 Check when copying object list
IQSM0003 Serial numbers, user exit for additional data <<<----
IQSM0004 Serial numbers, user exit after exiting the serial screen
IQSM0005 Serial numbers, user exit in the update <<<----
IQSM0007 Serial numbers, user exit for goods movements
IQSM0008 Serial number character string check
PeteA