You can absolutely have decoration added after the harp is finished, I’ve done this 3 or 4 different times, Altho if you ask a builder they prefer soundboard decoration to be done before the final finish is applied. Find an artist whose work you like and have a chat with them. Sometimes they get a little nervous about doing a very fine harp, since it’s unusual for them, but maybe an old beat up one would be less intimidating for them. One artist used acrylics, and another used oils-all came out beautifully. If I was more clever I’d post some pictures. Also, I wouldn’t recommend covering the whole soundboard with paint, might affect the sound.