Methods, systems, and computer program products for converting an object of one type to an object of another type that allow for the runtime operation of the conversion process to be altered or customized. The conversion may occur within an extensible serialization engine that serializes, deserializes, and transforms objects of various types. The runtime operation of the serialization engine is altered by one or more extension routines that implement the desired customizations or extensions, without requiring replacement of other existing routines. Based on type information, identified for an initial object, the object is converted to an intermediate representation which permits runtime modification, including modification of object names, object types, and object data. The intermediate representation of the initial object is modified in accordance with extension routines that alter the runtime operation of the serialization engine, and the intermediate representation is converted to a final object and type.