Board Thread:News and Announcements/@comment-1820055-20150223075709/@comment-26062182-20150223104701

119.238.132.227 wrote: ※「熟練見張員」は、大変ですが【菱餅】を沢山集めることでも入手可能です.

I think that this means that we can get the "Skilled Sentries" by gathering lots of Hishimochis All the text says is "The skilled sentry is serious, gathering many Hishimochi can make it available.", So I would say you are correct. Can't hurt but try to gather hishimochi's.