In my opinion this is already a great speaker set. My old roommate had this kit and we used it in the livingroom for several years for music, movies and games. The response curve is not flat for sure due to slightly boomy sub, small satellites and one way design on the satellites. So I do kind of get why you'd have this idea.
BUT, here is a curve I found of the Z5500 (orange line in left graph is the original I believe).
So they do drop off SLIGHTLY at the higher end, but I really don't think this should be a big deal. On a side note: The satellites really go low and so does the output on the amp to the satellites, I remember playing Jurassic Park pretty loud and we were right at the limits of the satellites as far as lower frequencies (seems like they even got down to 100-120Hz ok, really surprising for such a small speaker, I think those drivers were really powerful and well designed).
Anyway, I just swapped out the three front satellites for some bigger speakers and connected them to the amp built into the sub. Man they sounded great. Full sound from big speakers. Center was my JBL S-Center (3 way, dual 5.5-inch woofers, 150 watts, 80 to 20,000 Hz freq resp). L&R were my Bose 701 towers (dual 8" woofers, 400watts). It sounded so good. Deep sound even without the sub. The amp was putting out power at lower frequencies and the center, L and R had large full sound.
So, my suggestion would be maybe to upgrade just those three speakers with something slightly larger, 2 way design, keep everything else the same. It made a huge difference in our setup, really impressed me.
Still those Logitech satellites and their drivers were really good in my opinion, I wouldn't add the tweeters myself, but you might just try adjusting them, because they might be a bit directional being one ways.