https://www.db.yugioh-card.com/yugiohdb/faq_search.action?ope=5&fid=22941&keyword=&tag=-1
Adding material is a effect that affects the Xyz Monster.

https://www.db.yugioh-card.com/yugiohdb/faq_search.action?ope=5&fid=19250&keyword=&tag=-1
It still applies when the Xyz is face-down.