Go into account settings for each account. Down towards the bottom is an option to choose New mail ringtone.
Is that what you are looking for?
If you put the files in /sdcard/media/audio/notifications, K9 (and other programs that allow custom notifications) will see them.
There's also /sdcard/media/audio/alarms and /sdcard/media/audio/ringtones
You make have to create the folders if they don't already exist,.