The prefab is almost certainly .erf format, which the Toolset will import for you into your module.
Worth noting that custom content used in the prefab will need to be in your module too, in order to have it work correctly.
JFK
edit: Looking at the prefab for Mountain of Power, it uses a version of CEP and a Mountain Pass (tileset, probably). You’ll need to download those and edit your module properties to include them. There appears to be an .erf file for importing the area prefab, and also 2 more .hak files you’ll need to attach to your module like you do for the CEP and Mountain Pass .hak files. One of the .hak files looks to be custom placeables, and there’s an .erf to import that will put those placeables on your custom palette in the Toolset, if you want to build with them. I don’t believe you’ll need them if you only want to use the prefab as is, though.
I saw the Import… option under the file menu but it was grayed out. Once you sent me there it finally clicked that I had to start a new module with a placeholder area before the Import & Export functions were available. Basically, I was looking for the prefab as a module in the download, & that’s where I went astray.