Creating custom numbering system

Greetings to all!
I have the need to create a very specific numbering format. I 've been searching all around the Internet, but didn't find anything, even remotely close, to waht I'm looking for.

By default, InDesign has very specific set of numbering formats, numerals, latin numerals and latin letters.
I want to create a numbering system that uses greek letters, not latin.
Does anyone know if this is possible and how?

Keep in mind, I am looking for a permanent solution to this. I am not interested in work-arounds with paragraph and character styles, I already tried most of these. I, even, tried to manage a character-style based, GREP replacing routine, without any success.

Fun-fact: in ancient Greece, there were no numerals, but they used the letters as numbers too.
In many cases, greek writers prefer that style of numbering lists, thus the need for a permanent solution.

Thanks in advance!

Edit:
I must specify something important. I need to have total control of the creation of the numbering system.
That's because, "numbering" with greek letters doesn't correspond to their "numeral" value.
For example, the number 5, is the fifth greek letter "ε", but then, number 6 is an old "ligature" "στ", which is NOT included in modern greek alphabet. So, the 6 letter "ζ" of modern greek alphabet corresponds to number 7.