I have done this. I fitted a sub by buying an adaptor from autoleads. Its a Speaker to RCA adaptor, think its about £7.99, all you do is take feed off speaker wires, and it converts it into an RCA white/red plug output ready for the amp to be plugged in to. It then means you can then upgrade your stereo later if you like.
Does job ok for me, as Renault clio speakers are ok. Just headunit doesnt put out as much power, hence you might want to upgrade.