There are drivers and add-on super tweeters that will provide more HF extension. However, the stock driver is capable of providing quite remarkable performance. Unless you have done some of the following you are likely to be rather disappointed in an add-on.

1) solder all internal connections.
2) replace the high pass caps (stock mylars) with film types (like Solens).
3) consider upgrading your speaker wires (external and internal) and interconnects.
4) consider upgrading your source.

Items 1 and 2 are relatively inexpensive and can yield substantial gains in extension and resolution, provided that the speakers are receiving a good signal (hence items 3 and 4). A friend of mine has a pair of stock 4 Pi's and you can easily hear improvements in amplification, cabling and source. Whatever you do here-no great harm. Have fun and enjoy the music.

AstroSonic