Could I store my opened yeast in the freezer? If so, how?

Yes, dry yeast from an open bag can be stored in an airtight container and kept frozen. On the other hand, unopened bags ought to be kept in a cool dry place, and not frozen.

Anytime instant dry yeast is stored in the freezer it is important to only measure out the amount needed and only allow this amount to sit out until it has reached ambient temperature.