I often gets the same problem: Someones creates a nice bitmap font for me in form of a sheet nicely formated into a png, like this:
...and i need to create the angelcode .fnt atlas for using it (into Unity, libGDX, or countless other engines).
Yet, every time I google it, I stumble upon dozen of tools that create .fnt files from rasterized TTF fonts. (BMFont, Fontbuilder, etc.) Correct me if i'm wrong but they don't support exsting images, they always start by generating one from a vector font.
I end up semi-manually creating the file with an ugly script (the format is quite simple), but it's incredibly tedious. Surely there is a much better way to do that? Especially for monospaced fonts which are very easy to partition...