We are using Unity 2017.3 and need to switch the build setting to Mac OS X, however, Windows is the only option. From what I have been able to find through research, the only way to add Mac OS X as a build option is to reinstall Unity and select it as an option. Is there any other way to add Mac OS X as a build option?

Yes there is. All you have to do is run Unity installer again, and select any other platform support you want and it will only install those modules and won’t affect the base Unity application.