Kindly refer to the FM documentation for the EXTENSIONIN parameters. It says clearly that the fields in the BAPI append structure can only be of Character type.
The structures BAPI_TE_<NAME> (<NAME> = MARA, MARC, MARD, MBEW, MLGN, MLGT, MVKE) and the relevant checkbox structures BAPI_TE_<NAME> (<NAME> = MARAX, etc.) must first be extended by the customer to include the fields required. The standard structures contain only the corresponding key fields. When including new fields in these structures, make sure that the field has the same name as the field in the database table. In addition, the fields in the structures BAPI_TE_<NAME> may only be of the type CHARACTER. The data element BAPIUPDATE must be used for the fields in the checkbox structure (except for key fields).
Thanks,
Richa