is the end game to add a sub, or? If so, this may help
http://9thcivic.com/forum/threads/sub-install-moar-power.6085/
To answer the initial question, as far as I know there is not direct plug-n-play solution.
Let alone even a harness available from Metra or otherwise going the direction that you want to.
So I cheated and made one myself, using the pin out guide provided by immajackuup.
I purchased a Picasso Nano 5 channel amp, and a used Si amp w/ pigtails. Took the Si amp apart to get at the harness connectors
and use that to make a custom harness that allowed the amp to be wired in no splicing, cutting or anything.
This left the tweeters on the stock amp, but that could easily be changed with some component speakers, etc.
Just had to run the power, and stash the, unfortunately large cable harness that I made.
Made a custom bracket to hold the amp under the passenger seat....
...And aftermarket amp + all stock headunit.
Granted I upgraded everything soon after as the amp was "too good" for the stock speakers.
Also got an 8in sub in the stock location. But that's another story.