I would just get component speakers. Mount the tweets in 460 pod and go full range in rear
This is what I did. The 5x7/6x8 component sets were pretty expensive. I settled on a 6.75" component setup and used adapter plates. I went with Alpine because the crossover is inline and did not need to be mounted like other setups.
The problem with adding tweeters to existing 2 ways is you have to make sure the total impedance is within the range of your head unit. A typical scenario would be two 4 ohm speakers in series would be 8 ohms and ok for most head units. Wired in parallel you would get 2 ohms which would fry most head units. Check your head unit specs and Google for more explanation.
Also with adding tweeters to two-ways, you may have a very strong high end. You should probably be able to compensate and reduce treble on the equalizer but then you'd have a very reduced treble in the rear.