How can you change a Windows Movie Maker file to an ISO file?

You cannot change a Windows Movie Maker file (.MSWMM) into any other file format, including an ISO (which is simply an image that includes all the data of files contained on an archived CD/DVD, or other disc format).

Publishing/Finishing a .MSWMM is the only way to 'mix-down' (export) the file's audio/video properties to select Standard formats.