I know this thread is pretty old, but this question never gets old…
in Windows 10 you can add a greek keyboard to your system. Then use On-Screen-Keyboard (osk), switch between your local language and greek keyboards. This is not very fast, but the more you use the easier it gets:
How to use On-Screen-Keyboard: click on start button and type osk and hit enter. The On Screen Keyboard will appear on your screen.
How to add a greek keyboard to your system: click on the start button and type “language”. the “Regions and Language Settings” menu will appear. click it. A new screen will appear. Click “Add a Language” and choose Greek.
Before you move on, you need to make sure that on-screen keyboard has the focus, so click on it. then…
At the end of your start menu bar, near the time and date you will see an abbreviation of your local language (e.g. “ENG” for English). click it. A menu will pop up with the list of keyboards. click on the Greek keyboard. and notice that the On-screen Keyboard buttons change to greek letters.
Click on OneNote (or any other application) and start typing greek text.
Once done, change the keyboard to your local language.
Warning: if you turn off your computer before you change language back to your normal language, you will probably lock yourself out because your password will be in Greek and it won’t allow you to log in until you change the key board back to your normal language. This always happens to me.
enjoy.