Letter-based Numbering system

Hello,

I am trying to replicate the coptic language numbering system with uses letters instead of number (with an abbreviation mark). How can I can set up that in an opentype feature?

Info about the numbering system:

1-10 (there are ten letters

11-19 consists of the letter for 10 and than the letters for 1-9 then 21-29 consists of another letter (different from the previous) for 20 and then the letters for 1-9