If by "add a new letter" you mean the normal "Koch" training, there is a simple way to do it.
The program has two letter sequences programmed:
1) LCWO (KMURESNAPTLWI.JZ=FOY,VG5/Q92H38B?47C1D60X)
2) G4FON (KMRSUAPTLOWI.NJEF0YV,G5/Q9ZH38B?427C1D6X)
Under the option named "Characters sequence", select either "G4FON sequence" or "LCWO sequence".
Then under the option "Koch level", select which level you are at. For example, selecting "S" (with the LCWO sequence), it will play letters "KMURES".
If by "add a new letter" you mean doing the Koch training with your own custom sequence, you have to select "Chars from custom string" in "Characters sequence". Then the program will play characters chosen from the custom string. In this case, if you want to change the set of active characters, you have to add/delete them from the custom string.
Actually, more than allowing user defined progressive sequences, this feature was thought to allow definining small subset to heavily train "hard" letters.
Let me know if there is any further feature you would like to see in one of the next releases.
73 de Davide IZ2UUF
Thank you very much. Just the information I needed. Keep up the great work. 73