ok first you should still have rear L R channels on your HU, if you pluged into to RCA sub out put. you just dont have rear speakers in the back to play,
second, your way speakers all ready have "tweeters" built in thats why there the 4 in 4 way. the other three are just tweeters.
third, you can bring the rear channels up to the 4ways and have the front run the tweeters. but to be honest you cant fully power the the 4 ways adding more draw may not help you. and if you install the tweeters youll set up will have 8 tweeters, two speakers and a sub. thats not a very balanced sound