Hi Pratibha,
Thanks. It seems obvious that the transferred fields should be seen in the Customs Document (Sales Order replica).
You should use the BAdI described by Thomas, but you should use the Extension structures, rather than appending new fields to the ERP transfer structures - that's what I meant by "There are technical workarounds for the transfer of additional fields to GTS". There are corresponding BAdIs (documented in the IMG) to copy the fields into the Customer appends in the standard structures.
And you will also need BAdI /SAPSLL/BADI_CDOC_EX_UI_CON to add new tabs in the document.
Good luck!
Dave