When installing Unity on a Windows machine, you are allowed to select "iOS Build Support" as one of the components to install. However, it is not possible to make an iOS build from Windows. Why is this package available on Windows? What does it offer?
As far as i know when you build for IOS on windows it will generate an Xcode project that you then need to build on mac.
As you can't make IOS bundles on windows the only thing you can do is to make the process better by not having the requirement of installing Unity on both windows and OSX. You could set up automated tools which take the output from unity and automatically build the game on a mac so that you don't have to go though as many hoops. See Jenkins.
Hope this helps :)