is there any way to wire a sub into the existing bose system?

 

1) Yes

2) Of what car?

3) Usually most installs consist of using a speaker level or line output convertor (LOC) on the Bose woofer speaker wires into an amp, OR using an amp that takes speaker wires in. If you want to drive the amp with preamp in, you are seriously better off replaing the amp and all the speakers.