Hy Ashish,
as far as I remember there are three entities you need for Import/Export of BOM's.
Bill of materials Header (is current)
Bill of materials Versions V3 (is current)
OR
Bill of materials Headers and Versions V3 (is current)
and
Bill of materials Lines V3 (is current)
BOM Headers and Version Entity is available for "open in Excel" from the BOM form.

For Bil of material lines i did not see an option for "Open in Excel", but it should be importable with the Excel addIn.