Microsoft Expressions to Dreamweaver

Dreamweaver is a glorified code editor.  It can open any native web file but it doesn't have support for ASP or ASP.net.  So if your site was built on that coding environment, you would be better off using MS Visual Studio Code.  

In terms of Windows or Mac, the computer platform used to create your web files is irrelevant.  Code is code.

Check the file extensions for the pages in your Expressions created website. Unless the extensions are proprietary to Expressions, any HTML, PHP, et al documents should be editable using Dreamweaver. Follow the rules to define your website in Dreamweaver and then copy the Expression files to that folder.

Chris