complaint creation with reference to ECC documents

Dear Friends,

we have an requirement for implementing Complaints scenario. we want to create a complete with reference to the external document. we are doing billing process in ECC.

as we are doing billing in ECC, what are the documents we can use for referencing.

also let us know the process of how to get the external documents from ECC to CRM system?

Thanks in advance

Regards,

Anand P