Game Font Editor is a really useful tool, and I also want to point out that it can be used to copy font symbols from the clipboard, for example from different mods. It even allows copying from large resolution to small resolution font maps while doing the resizing automatically. This wasn't immediately obvious to me but can be really useful.
The only thing that it does not support (to my knowledge) is extending the game font files to create additional slots. In that case the approach from dbkdlk still needs to be followed. However it is sufficient to add more "white boxes", Game Font Editor will detect them and you can paste icons into them as usual.
Care needs to be taken when extending game font files in this way. There is some magic going on in the backend when mapping the file to actually existing font symbols that can get disrupted by adding slots. This can usually be worked around by filling more unused slots in preceding rows with icons instead of keeping them blank. I usually have to do this by trial and error, if someone can shed some light on the rules behind this it would be helpful.