How can I adapt rice cooker steamed fish to cooking in a pot?
Easy! I steam fish about 1x per week with a bamboo steamer. You need to take advantage of the steam rising from the pot which can steam your fish in exactly the same way as the rice cooker - so you get a Bamboo Steamer like this one (available in tons of different places, including cooking-supply stores, Asian Foods stores, etc): http://www.amazon.com/Norpro-Deluxe-3-Piece-Bamboo-Steamer/dp/B00005EBH8

This steamer is basically open at the bottom (with bamboo slats) so you set this on top of the boiling pot, and it fills with steam, cooking the fish. It's recommendable that you wrap the fish in wax paper (I use baking paper in a pinch) and that will keep it from sticking to the bamboo.

Hope this helps! I recommend buying a bamboo steamer like this one - it's a common kitchen item that you can use for a ton of steaming - i.e. veggies, broccoli, fish, etc. No need to even be cooking rice - you just place this over a pot of boiling water, and it works great!