Some amps have a passthrough for the input signal, and you're going to need power.

If your power wiring is insufficient, you'll need to either run an additional wire to the new amp, or upsize your existing wire.

If your amp doesn't have a passthrough, you can Wye off the existing RCA's or run new ones from the head unit.

The problem with the Wye connections, is that you'll need a crossover of some sort, either built in the amp or added on. You'll also loose your fading capability in the head unit to set your subwoofer's level relative to the full range complement.

If you have any doubts in yourself of doing this correctly or safely, PLEASE let a professional do it for you.