Yes. To do this, add water to your reservoir, being careful to not over-fill (fill no more than ¾ full—water expands as it freezes; it can burst your reservoir if you over-fill it). Purge all air and then lay your reservoir flat in the freezer overnight. Remove and enjoy a steady source of cold, refreshing hydration as the water melts!
Also, did you know that you can occasionally skip cleaning your reservoir by storing it in the freezer? It’s as simple as rinsing the whole system with fresh water, then placing it in the freezer until the next use.