Personally, I start from a document format (doc or odt) and I save it in HTML format before using Mobipocket Creator. The greek letters are saved in the HTML "α" mode.
If you use polytonic greek letters, that would be not enough, and you will have to open the HTML file in Notepad and save it with the UTF-8 encoding before uploading it to Mobipocket creator.
The resulting HTML UTF-8 file can be uploaded to Mobipocket and/or edited to obtain your desired formatting.
Here
https://www.mobileread.com/forums/sho...d.php?t=163021 you can find the Odyssey formatted by me using this technique.