i am a licensed electrician....don't do that.
all cords from appliances are purposely,about 6 feet long (at least in america) i think it is for voltage drop....long story short, the wire for the plug of your appliance is small, and the further from the power source you get, the more resistance on the wire, which makes it harder for the electricity to get to your appliance. (or apc in this case) the only reason extension cords work, is the wire is bigger and therefore can carry electricity further.....
if i was you, i would get an extension cord....its the safest option
if you do splice something together,(which i don't endorse, especially as a layman) DO NOT use gaffa tape!!!!!!!!
get some "butt splices"
http://www.grainger.com/Grainger/3M-But ... Pid=search
you will also need a crimp tool
http://www.delcity.net/store/Multi-Purp ... 534578.a_1
and some decent electric tape...(3m scotch super 33+) because its rated for what you want to do.
all of this stuff can be found at your local hardware store for pretty cheap.
strip the wires of your plug, and stick them into the ends of the butt splices and crimp....then tape it all up really good.
two things.....if you add to your plug, don't go crazy on the length, if you need more than a foot or so, get an extension cord...because you could have problems with your apc (due to the mentioned voltage drop and resistance) also when you cut both of your cords, im not sure if it is the same in europe, but most cords in america have the neutral marked somehow. (the neutral in two wire plugs is kind of like the ground, it carries electricity back to your panel to ground....so if you switch these it could reverse polarity in the apc (because plugs can only go into receptacles one way(again, at least it is that way in america now a days)...so if you have these wires switched you could be sending the power to the wrong place in your apc)so... on your apc cord, you will notice, one of the wires has white dashes the length of the cable...that is the neutral.
sorry for the "lesson", but electricity is mad dangerous son!!!! and if you use gaffa tape...you will most likely cause a fire or shock the hell out of yourself....
good luck.