[el-Odysseas]

I guess that you can insert the key manualy:
Write "Cb" on the key-field yourself.

About the "CP", can it happen, that you entered it wrong at some point (for some other piece) and that's why you have it on your list?
Whatever you write on the list (on any field for that case) it's being saved on the list.

Try opening an existing song in C, open the editor and insert in the key-field beside "C" a different symbol than the usual (for example instead of C, write: 
C-major- or C+ ) and save it.
Then open the editor for another song and the new inserted symbols/keys will be on your keys-list.

I hope that helps,

el-Odysseas

P.S.: About the missing Cb key, once you enter it once yourself manualy, like described above, it will be found next time on the list.
You can also delete the "CP" from the list.