Is it possible to add speeds to single-speed bike? I found this cool cheap single-speed bike on amazon http://www.amazon.de/Fixie-Singlespeed-Viking-Farben-Auswahl/dp/B00BCKQ8AS/ref=lp_235171011_1_1?s=sports&amp;ie=UTF8&amp;qid=1422793944&amp;sr=1-1 

However my problem with it is that it is single speed &amp; I would like to have 27 speeds. Is it possible to do that manually? i.e. to buy speeds and add it to the bike? I would why they didn't do it themselves!
If you want a 27 speed bike then buy one.  It is not cheaper to add speeds to a single speed or fixie bike.  Lots of stuff is not compatible including axle spacing and real wheel.