If a literary work is partly added by persons related to the original writer, it would not be called "forged writings" (as for "The Tale of Genji," though this is a sample from ancient Japan, some volumes are doubted to be written by other person than Murasaki Shikibu).
縁のある人物が一部を加筆するといったケース（時代を遡った日本の例になるが、『源氏物語』は一部の巻が紫式部以外による執筆を疑われている）などは、普通贋作とは（無論偽書とも）呼ばない。