Hi Senthil,
You cannot add new items aferwards to an existing Customs Declaration created from a feeder system transaction.
Instead, you should make sure that all Inbound Deliveries have the BoL number assigned before you create the Import Declaration - then all of the associated items will be correctly included.
If you need further details, let us know which GTS version you are using, and whether you create Declarations before or after Goods Receipt has been posted.
Regards,
Dave