I would like to define a new keyboard layout for KDE 4. In particular I would like to take the standard British layout and add Polish characters for the combinations Alt-Gr + letter. How can I do it?
KDE simply uses the X server's keyboard mapping facility (XKB). This is more complex than it looks, and definitely more complex than it should be. Sigh.
The quick and dirty answer would be to combine the files in
/usr/share/X11/xkb/symbols/pl into a new file, but you also need to provide information via XML for front-ends (like KDE4), and you'd probably have to read a few hundred pages of documentation.
But you may be able to get away with less: have you seen this tutorial on making a hybrid de/pl keymap?