Input Farsi / Persian on Latin keyboard

I have a computer with a german keyboard (i.e. latin characters) and want to input Persian/Farsi characters. Since I do not know the Farsi keyboard layout very well, I would like to use an input method, that is able to transliterate latin phonetics to Farsi characters. I imagine pressing a magic key combination to switch into a compose-mode, and then typing something like "sh" to get "ش", or "p" to get "‏پ".

Is that possible with Ubuntu?

I tried to add Persian language support to Ubuntu (KDE Systemsettings / Regional Support / Language), but cannot figure out how to add an input method of the sort I described (I only find keyboard settings for the layout - which is not what I want to change).

Since I read on Wikipedia about IBus (and it being the default foreign input method in Ubuntu since a while), I tried to "apt-get install ibus ibus-qt4 ibus-m17n", and configured it according to [1] (using "Persian / Isiri - m17n" as the input method in ibus-setup). This allowed me to switch into Farsi mode using Ctrl+Space, but when I type, it just seems to have changed the keyboard layout to Farsi - I cannot combose the Farsi characters using a transliteration method as described above. I assume I am using the wrong IBus engine...?

[1] http://greenmang0.wordpress.com/2010/10/15/ibus-now-write-in-your-mother-tongue-%E0%A4%AE%E0%A4%BE%E0%A4%A4%E0%A5%83%E0%A4%AD%E0%A4%BE%E0%A4%B7%E0%A5%87%E0%A4%A4-%E0%A4%B2%E0%A4%BF%E0%A4%B9%E0%A4%BE/