OLM files are not supported in Mac mail. Mac mail imports in a different file format. Moreover, there is no addon facility in Apple Mail or Outlook that can convert the same for free. There are, however some paid tools that can help you convert OLM files to Mac mail format. I have used products by Gladwev in the past. They were considerably fast and excellent at keeping the mail contents and the folders in the right place. Their tool is named OLM converter Pro.