The Warning is a warning, program will continue (I already wrote you can Customize the message type, e.g. convert it to space, read also 2179995 - Confusing warning/error message ME 887 and 1918804 - Error ME887 when using ExtensionIn on BAPI_PO_CREATE1, BAPI_PO_CHANGE, BAPI_PR_CREATE, BAPI_PR_CHANGE or IDoc for reference)
capturing all character like data types and moving them to structure
if there are only character fields you don't need the BAdI, the BAdI is useful for non character types, if you get the warning some fields are not character fields (the actual final structure, not BAPIREX, of course)
Not all fields are being transferred through extensionin
So check again your BAdI implementation, and forget the Warning.
Regards,
Raymond