I'm looking for an acoustic pickup to use with percussive fingerstyle, something that will pick up body slaps and pops. I came across the K&K trinity western http://www.kksound.com/trinitywestern.html which was highly recommended for this. The only thing is, I already have an onboard preamp on my acoustic, and don't really want to change it. Would this pickup work with an onboard preamp that's already in place? Or would I just bypass it completely? Or, if anybody knows any other pickups that would work well for percussive fingerstyle that would be useful. Thanks.
Any pickup type that's a microphone will do this. Microphone acoustic pickups are fairly common.

You don't say what type of guitar it is, but let's assume that the onboard pickup is of lesser quality than the one provided with the K&K. I'd use the one provided with the K&K regardless, since it was designed to be used with their pickup. There's also a slight chance the onboard preamp isn't compatible with the new pickup.
Sorry, you are correct in assuming it has an onboard preamp. This helps, but would I just disconnect the preamp and leave it and the original pickup in, or take the pickup out and leave just the preamp? Or does it even matter?
I'd leave the old preamp in place, if at all possible. If you ever sell the guitar, you'll probably want the original electronics installed, or at least available. I'd also leave the original pickup installed. As long as it's not causing any issues, why remove them?
The only reason I'd remove them is if they don't disconnect from the input jack. The western has its own input jack, but I'm assuming they're probably a way to disconnect the current pickup from the old input jack. Thanks for the help.
What you could do is just loosen the old jack and loop the cable up inside the body of the guitar and use a tie strap to keep it from bouncing around. That option leaves it available to reinstall in the future. If it's like a Taylor, where the 1/4" jack is part of the strap button, then you may have to find another option.

You're welcome. Best of luck.