Prathiba,
If it is a return sales order then don't you think it need to be mapped as inbound document in GTS?
User exit I have provided will bring all the data from the respective document and you can change the content for the structures whcih are getting mapped to GTS structures.
Best thing you could do in this situation is - Ask your Abapper to put a break point and show you the content being passed to GTS and you can figure out which fiedls you have to map additionaly into GTS structures.
Once the data reaches GTS, you can remove the newly mapped content and map them to the necessary fields i.e. document types into the declarations.
Regards
Raj