The issue here is that WA has a lot of salmonid populations in danger. They are more concerned about releasing a disease into wild populations than anything else. Our river systems are full of dams and that has caused populations to become more isolated, inbred, and less able to fend off foreign diseases. I understand that a pond seems very harmless, but it is against the law here. If you really want to try it, you can contact the Washington Department of Fish and Wildlife and they will send out a fish biologist to look at your pond and decide if it is an appropriate habitat for trout. You have to fill out an application, pay a fee, and wait to see what the biologist says. It is possible that you will have to apply for additional permits as well depending on if any other changes need to be made. I encourage you to pursue that direction if you want to do this project because a fish biologist looking at your pond will be able to determine better than us if your pond is a suitable habitat for native wildlife.