It seems there is no import option for textures as 'BitmapFonts'.
I set them up via gd script.
var font = BitmapFont.new()
font.add_texture(texture)
font.set_height(9)
font.add_char(KEY_A,0,Rect2(Vector2( 1, 1),Vector2(7,9)))
...
ResourceSaver.save("res://fonts/font.tres",font)
You can use 'add_char' to add characters. The first argument is the ID of the texture, the second argument is a 'Rect2' with the position and size of the letter in the texture. If the characters are aligned in a grid in the texture file you can use a for loop (KEY_A+1 = KEY_B and so on). KEY_A+32 is the capital A.