What are you powering those speakers with? If the sound is coming from the rear only you might need more power up front or turn the rear 6x9's down (using the fader.)
You can easily plug those tweeters in as long as you're not driving the amps into clipping all day long aka, cranking it to 11.
If you decide to, wire them into parallel with the entire new polk speaker system (If they're coaxial, just wire in parallel to the only terminals on them, should only be one.) Cut off the special connector and spice it in. All this was suggested by other people so don't thank me, thank them.
Its not that its a bad idea, its that there are better ways of setting this up. But if you're not looking to blow the doors off your car like the guy in the maxell commercial then you should be okay.
YouTube - Maxell ad