I did have the Highline system but I have replaced it with an after-market stereo. It is a full HK set-up and I was after a better bass quality. I do have a after-market sub and amp but I was trying to find out if I can integrate the existing one first before I remove.
You could use the same feed that you use for your aftermarket sub, to drive the HK sub, unless the aftermarket has it's own built in amp. If so, then you just need a two channel amp. More involved is to use the six channel HK amp as well.