The ICO.dll (or ICO.so, or ICO.bundle) is created during the build process. All of the code Imager requires to support ICO is part of Imager (most of the format support is in ICO/msicon.c).

The structure of images inside an ICO file is similar enough to BMP that you could probably write a simple script (or module) to do the conversion. One issue is that .ico doesn't allow compressed images, but you could test for that, since you need to parse the bmp header for the image size anyway.