When Jana and I were cutting the video of our holidays in Australia, we needed a font resembling the australian outback style. We chose "Roughage" as it is kind of woodwork-like. The only drawback was that it only has the most commonly used letters, which meant that the German "Umlauts" (äöüÄÖÜ) are missing. This led to the not so satisfying cases in which we needed to replace them with their transliterated couterparts of "ae", "oe" and such. It was ok, but I wished the font would actually have this letters.
Today I took the opportunity and looked for a font editor and found FontForge. After loading Roughage's ttf file I played around a bit and found out that you could create references of characters and use them to create new ones. This is ideal since you can create an "i" from the i without the dot "ı" and a single dot "̇".
So I copied the "i", removed the part below the dot and had my dot without anything else. I copied the do twice and had character 0x0308 "̈" that could be used to create my umlauts. Now I selected the ä character field, went into the element menu, build, build accented character - voila, I had an ä in the font. The same for the other 5 characters plus some adjustments for the dots, and I was done. If I knew how easy this is, I would've fixed all my fonts earlier.