So it seems that not even the OP is very interested in it... :/
I researched even further, and there is (was) an application with a GUI called Keymapper, present also in Software Center, but this doesn't work. It installs but then there is not trace of it anywhere.
sudo apt-get install gives the same result.
Honestly, I hear everywhere that Linux is superior, but to modify my keyboard layout in Windows 7 took about 10 minutes starting from the download center in the microsoft website, to having my custom keyboard layout installed and perfectly working. With Linux I am trying almost since last week, and I can't find anything relevant. I don't feel that compiling a layout file is the way to go... we are almost in 2013, there must be something easier, even in Linux!
Why Linux is sometimes so ubercomplicated? To scare off n00bs like me?
Please, see my situation: I am a translator, I need to use 4 languages, and the only differences are just few letters with their accents. It is plain stupid to have 4 layouts installed and switching them continuously, not only letters' positions change, but also all the punctuation. As it is my job, having to switch continuously from one layout to another lowers speed and consequently my productivity. Without considering every time that one layout has been forgotten active instead of another...