Ravi,
It does not have to be only Proforma Invoice to create export declaration, However depending on your business scenario.
You can clear export customs using proforma however commercial invoice is must at the time of clearing goods on the receiving side.
If you can generate commercial invoice by the time you file export declaration, you can absolutely use it however please check if that can happen in all cases otherwise you have to fall back on proforma.
Regarding the configuration:
ECC:
Check -SPRO - SD - Foreign Trade Customs - SAP GTS plugin - Control Data Transfer - Configure Doc types -Check the SD0C
Here you can can see which Billin doc types are configured.
Once you find them, Check the Feeder system doc types mapping in GTS.
Please do let me know if couldn't get there. Thanks.
Regards
Raj