I wanted to use an icon in a windows exe-file. I extractet the ico-file by using a tool called wrestool. It was found in a package, which was easily installed:

sudo apt-get install icoutils

I extracted the ico with this command:

wrestool -x --output=. -t14 file.exe

I could then easily convert the file to whatever, and place the file in my wanted location /usr/share/icons