First thanks for any help in advance.

I already have 4 speakers installed and am looking to install 2 more in the lowers.

My question is do I need to install an additional amp or can I piggyback off another channel.

 

That depends on what amp you have and how many ohms the speakers are.
If your amp is 4 ohm and stable down to 2 ohms, and your speakers are 4ohms then you can run 2 pairs of speakers in parallel which would be 2 ohms then the other Pair of speakers can just be run normally off of the other channel.
But if all of your speakers are 2 ohms you would need an additional amp since if run in parallel they would bring it down to 1 amp, and that would ruin the amp.

 

What I (actually not my bike) have is a Memphis Car Audio 4 channel amp, 4 focals, and have an extra set of 6.25 hawg wired I would like to put in the passenger pods. I'm not sure what ohm the focal and Hawg wired speakers are. Need to do some research.

 

It looks like the Memphis amps are 4ohm and 2 ohm stable and the Focals are 4ohm speakers as are the Hawg -wired speakers.
You can run the upper fairing and lower fairing speakers in parallel off of one channel and then the rear speakers off of the other channel, this would be the best way to wire them up.