As far as I know, you cannot freeze crystallized flowers for later use. Depending on what crystallizing recipe you use, you can layer them interspersed with waxed paper in an airtight container. I believe this is possible if you use gum arabic for your sugar binder.
I, however, just enjoy making them as I need them by simply coating the flower petals with reconstituted powdered pasteurized egg white & then dipping in superfine granulated sugar & allowing to dry on a sheet of wax paper. I've used this method with success on violets, rose petals, & mint leaves.