Can I add the kick panels that come in the newer berettas to my car?
You bet. They are NOT a direct swap, and you will have to work at it a while to get them looking like they came from the factory. Here's the basic procedure. Your gonna need to buy the kick panels, AND the 2 door sill pieces from a 91+ beretta/corsica. You can install them without the newer door pieces, but it will take more work to do. (Note: The kick panels can be out of either, I haven't checked but I assume the door pieces from either will also work.) You also need to keep your existing door pieces, because the new ones are shorter. So run some wire, then put the door sills and kicks in place. You may have to do some minor trimming to the kicks for them to fit in there. Since the new door sill pieces aren't as long as your old ones, you need to cut the back of your old ones off and reuse them. You can either try and overlap the two, or just have them meet. That's the basic procedure, it's not hard, but it can be a pain trying to make them fit in there.
Your other options are to either try and make a set from scratch, or try and fit a set made for a different car as I don't think anyone makes a specific set for berettas. Of course you could also put some speakers in the doors.