It can be done, but will require you to disconect your rear speakers, meaning you are left with front and sub,
what do you want more? Rear speakers or a sub?
If you want the sub then buy a line level converter, disconect both rear speakers and run the wires from 1 of them into the converter, this will convert it into a pre-out phono conection, this then plugs into the amp.
Some amps have speaker level inputs, meaning you miss out the converter and extend the speaker wires to the amps speaker level input,
these methods give a poorer sound quality though,
If you need all 4 speakers and a sub then get a 2 channel amp with speaker level input ( or 4 channel if you want to amp all 4) and run either front or rear speakers off the amp and extend there speakers wires to the amp, some amps have phono inputs AND outputs, you can then simply run a phono lead from the output to your subs amp.......