Bitmap font is actually in the default files. It's part of a DDS file. You can edit it, sure. It's data/gfx/gui
It's one of the font files, don't know if it was the eng one or not. Do note, and this is
important, you have to enable bitmap fonts in a patcher like abyss. This file is not read by default, and i don't think data folder mods alone will enable it, though i could be mistaken on that part.
There's your default one, but i inverted the colors, font's white by default, for visibility.
And there you can see where i edited it. Made the capital I have the horizontal lines, rather than look like a lower case L. The changes reflect in game appropriately, making it easier on me. So yeah, you can edit the default bitmap, but not like normal fonts. You actually have to draw the font. And each letter has a set space in the .dds file, if you exceed the selection range, then your letter will be clipped in game. I don't know if you can increase the dimensions or something to increase selection range, or if there's a file to edit the dimensions for each individual selection. So you probably can't make anything super fancy that'd still be legible in game.
You can always open the file in paint.net and use the font feature, type out your letters, get the size to be roughly similar to the originals, then just use the actual selection+resizing option rather than trying to get the closest font size through font features alone. Just make sure you have the desired font installed.
Think that's all my knowledge on the matter. Good luck messing with it.