such as National Instruments DAQ ones.
The arduino has 6 analogue input channels of 10 bits each, that's a reading of 0 to 1023 on each input.
how can I acquire signals with nevative values, and/or with values outside the max range?
You need some signal conditioning (extra hardware) in front of the analogue inputs.
is there an acquisition software so that I've not to write mine?
there is this:- http://arduino.cc/en/Reference/Firmata it puts the onus of gathering data onto something like a program written in Processing or other such language.
I need only a csv export of acquired signals.
You will have to do that in some language running on the host PC, like processing mentioned above.