Creating new key signatures

I need to create classical Turkish key signatures. I've found most of the accidentals I need, but I can't figure out how to create my own key signatures. Example, the Çargah pieces require a mirror-image-flatted B and a forward-flat-with-stem-slash D. When creating a score, we need the option to create the key signature as well as to select a key signature. That would really help! Thanks!!