In accordion scores, special labels indicate the treble shift. The capella font does not contain these labels, but there is a particular font for this kind of notation.
Here you can find the file acordeon.ttf for free download (please scroll to the very bottom of the page and click on "Zeichensatz mit Akkordeon-Symbolen").
As soon as you have installed the font via the control panel it will be at your disposal for text objects in capella.
Create a text object and select the font "Acordeon". To spare you some guesswork, here is the keyboard layout: