supplement

[1]	add (on/to) , extend , augment , complement
[2]	append , add on , affix

[1]	addendum , addendum , addition , postscript , appendix , epilogue , end-piece , postscript , appendage , extension , continuation , adjunct , annexe , appurtenance , accessory , codicil , insert , sequel , supplementation , (Technical) suppletion
[2]	supplementation
[3]	accessory , appurtenance , add-on