Carol, Yes, you can boil peanuts in a slow cooker.
The boiling time will vary, depending on whether you have green peanuts, or dried peanuts. Expect green peanuts to take about 1 to 4 hours to soften, and dried peanuts will likely require 24 hours or more. A friend of mine says he simmers them in the slow cooker for 2 days before they become as tender as he likes.
The cooking time will vary with the heat setting on your slow cooker. Add water as they cook, to keep the peanuts submerged.
There is some evidence that the amount of salt used affects the time required to soften peanuts, and dried beans. I usually boil my peanuts without salt, until they are about 75% as soft as I intend. I add the salt for the remaining cooking time.
Soak the peanuts in the brine as it cools. The peanuts become wetter inside the shell, and saltier, the longer they soak.
Boil them until they are soft, soak them until they are salty.