I don't think that is possible.
You will need the MM for Windows version for importing from XMind.
However, If you have only a few to several hundreds, AND you just need to import them just once...
Use / download the Windows trial version. It works for at least 30 days with full functionality.
Once you have imported them, you should be able to svae them in the same cloud space as you use for your WEB -Essentials version.
Adding on to Ary's reply, MindManager Mac also contains the XMind Import. Once you use MindManager for Windows and / or Mac to import and convert the XMind Maps to MindManager files, they should be fully compatible with MindManager Web.