Since there are quite a few combinations involving the Option key that haven't been defined, there is ample room to add what you want. Another program that will let you do the same thing is ResEdit. Mac Insight'' by Lon Poole contains a lot of good information and step by step directions for using ResEdit. In either case, you find the font you want to change and edit the character as if it were in FatBits. Fonts are fun.
Diacritics are accessible.
Whether you use the Mac's system file, buy a special font or create your own font depends on your needs. I've used them all. My preference is SuperFrench German Spanish. It does what I want to do as fast as I want to do it.