I do not believe there is anyway to import a mac image. It can't be imported via the GUI and the Resource Import tool doesn't have any option to specify an image as MacOS, and I don't believe you can leave that blank. The only way I can think to capture it without having to boot to it first would be to netboot and deploy the image manually (using the asr command) and then use the built in capture task to then capture the image. It's not pretty.