Hi,
I have no clue, how the fonts system in linux works.
All I know is that applications like abiword or xchat have very little choice, and I would like to change this lack.

What have I to do to accomplish this?

You can add your fonts coping it to /usr/X11R6/lib/X11/fonts/ directories...and running (inside the dir) fc-cache -f command
You need to create a new font index using mkfontdir
Your fonts directory must be listed into /etc/X11/XF86config file (take a look) and must have read permission (444)

mkfontdir does not work... command not found

I'll expand on what Obscure said.

There are two font management systems you have to worry about. Fontconfig has one, and X has one. You deal with the two of them separately.

ADDING FONTS TO FONTCONFIG

Fontconfig supplies fonts to your gtk2 and qt programs. First, put your new Truetype fonts into a directory. Then look for a file called /etc/fonts/fonts.conf. Read it. Learn where it searches for fonts, and how to add a font search path to it, but don't edit it. Instead, make your changes to /etc/fonts/local.conf. Then run "fc-cache -f -v" as root. The new fonts should be accessible now.

ADDING FONTS TO X

To add fonts to X, you need ttmkfdir. Put all your new Truetype fonts into a directory. Then go to that directory and type:

ttmkfdir > fonts.scale
ttmkfdir > fonts.dir

If you look in /etc/XF86Config, it will show you which directories X looks in for fonts. It will also become obvious how to add a "fontpath" to it. Add your new font directory, which contains your fonts and your fonts.scale and your fonts.dir, to your XF86Config's fontpaths. Then restart X.

XFree86 Font De-uglification HOWTO

http://www.tldp.org/HOWTO/FDU/index.html