Hi Praveen,
For BIGINT type variables you can use SAP's INTEGER64 data element. Create a variable and assign MATNR value to this variable when inserting to table.
Just don't forget that MATNR field is CHAR18 field and may contain non-numerical values. If there are non-numerical MATNR values, there is nothing you can do to insert those records to a table with BIGINT Material Code field.
Regards,
Murat Kaya