First, you'll have to find the file that defines your keyboard layout. In Ubuntu 10.04, these files are located in a directory called /usr/share/X11/xkb/symbols. Most files use a land code as a name, with the exception of some vendor specific definition files, and a few more generic ones.
The format of these files is pretty self-explanatory. Most files define a basic layout, which is the default one, and some variations. Each layout can include another layout from the same file, or from another file. Keys are defined by their row and column name, and can contain up to four values (in the simple case). The first value is the character you'll get when you just press the key, the second one when you combine the key with the Shift key, the third one when you combine it with the Alt Gr key, and the fourth one when you combine the key with both the Shift and the Alt Gr key.
In my particular case, I use a standard Norwegian keyboard, i.e. the basic layout in the file called no. Since I have to write mails in Esperanto from time to time, I need access to characters like «ŝ» (s with circumflex) and «ĉ» (c with circumflex). They are easy to produce on a standard Norwegian keyboard in Ubuntu: just press the dead circumflex key («^») first, and s or c afterwards. The circumflex key is called “dead” since it doesn't produce a character until another key is pressed. It's more difficult to produce «ŭ» (u with a breve)—in fact, as far as I know, it's impossible to produce it directly from a Norwegian keyboard.
For me, it makes the most sense to put the dead breve key («˘») together with the dead diaeresis key («"»), the circumflex and the tilde («~»). There is however no definition for the dead diaeresis key in the Norwegian keyboard layout file, so that's not where we have to make our change. In fact, since the basic Norwegian keyboard layout uses latin(type2) as its basis, we'll have to look in the latin file and see whether we can find the definition of the diaeresis key there. In fact, it's right the in the type2 definition, at the following line:
key <AD12> { [dead_diaeresis, dead_circumflex, dead_tilde, dead_caron ] };
Indeed, the dead diaeresis key is on the fourth row (encoded as AD), and the 12th key from the left. Now how can we change this definition to produce a breve instead of a caron («ˇ») when we combine the key with Alt Gr and Shift? In order to do that, we'll have to change the definition as follows:
key <AD12> { [dead_diaeresis, dead_circumflex, dead_tilde, dead_breve ] };
In order to keep the dead caron key, we could also change the definition for the AD11, the key that normally produces the Norwegian letter å, as follows:
key <AD11>{ [ aring, Aring, dead_abovering, dead_caron ] };
Once you've made the changes, save the file, and reboot your computer in order for the changes to take effect. When the system comes back up, you should be able to produce the character «ŭ» using the diaeresis key in combination with Alt Gr and Shift, and typing u afterwards.
