Do I need to thaw fish before steaming it? Can I thaw fish during steam cooking or is it better to leave the fish the night before in the fridge?
Thawing as part of the cooking process is safe.

If your fish is very thin, you may get good results.  The risk is that the outside will overcook before the inside is thawed and cooked through.

You will probably get better results thawing the fish in the refrigerator before steaming it.