Custom Fonts?

Of course you can create custom fonts in MacOS X. FontLab's FontLab Studio, its veteran font editor Fontographer, and several other excellent font creation tools can be found here.

There are a lot of really bad fonts created by experienced amateurs. The novice does not stand a chance. If you need a custom font, then you will be much better served by hiring a professional to do the job for you. If you can't afford a professional type designer, then select a commercial font that is close to what you want. You can convert your characters to outlines and tweak them to a closer approximation of what you envision.