you can connect them to standard radio you can buy special splitters from halfords go in there explain to them what u wanna do and they will get u the lead.
but tbh i wouldnt recomend it because the standard radio doesnt have seperate amp controll so in effect all speakers/sub would be working at the same watt power the best thing you can do it just split more speakers off the same speaker output i wouldnt worry about the sub if you want to keep the standard head unit
this has nothing to do with the question but it is interesting if u have a mk 7 fiesta look in the glovebox on the side walls of it it has coin holders in it interesting fact that not many people know