I want to make a casserole dish ahead of time that calls for thawed hash browns. Can I combine all of the ingredients, defrost the hash browns, and return the uncooked hash browns back to the freezer?

You can do this, but the quality of the thawed and refrozen hash browns will not be as crispy when refrozen and they will hold up for a shorter time period, no more than just a few days to a week. Frozen hash browns are flash frozen very quickly and this is hard to replicate at home, so the potato has a tendency to be soggy.

Another option you might try is to use a package of dehydrated hash browns. That way, you can add water to make how many you want, while the amount that remains unhydrated is still shelf-stable. These products can be found in the grocery aisle where the instant mashed potatoes are sold.