As long as the Windows and Mac installations of eM Client are the same version, you should be able to backup eM Client on Windows and restore it on the Mac.
Probably the easiest way, especially for cross-platform moves, it just to have all your data synced online. So for example if you use a Gmail account, all your messages, contacts, tasks and events are synced with the Google server. Adding the same email account to the Mac installation means that the same data will appear there without you having to copy anything between the computers. For this method the versions of eM Client do not need to be the same.
Even if you don’t use a Gmail or similar account, getting one just for the purpose of syncing your data for the move won’t cost you anything.
No, not import. Backup on Windows eM Client using Menu > Backup. Copy the zip file to the Mac. Restore using Menu > File > Restore. For this to work both must be the same version. It will not work if the version numbers are different.
Your best option is still to sync with a Gmail or similar account. Add the same account on both computers and the messages will be the same on both computers.