Thanks a lot, good that I realised and asked before doing 
Soldering a wire could be a simple solution, but if possible, I'd like to preserve the Arduino board for other projects, so I'm thinking to realise my prototype on a breadboard shield. This is why I initially thought of using the V_in pin.
I'll move to a double connector, putting the Arduino board inside the box instead of right behind the planel. Mmmh... this means I also have to double the USB connector if I want to access it from outside. Does someone know if an extension cable for that "square" USB connector (don't know the name) exists?