I don't think it's specifically about frequency range. Frequency range is covered. It's about the actual sound of certain higher frequency instruments that the ribbons to my ears handle better. Cymbals, percussion, etc. And add-on with built in cross over is a great idea. I wonder if anyone anywhere has that.
Yes, Onix has done that. Onix ERT (Extended Range Tweeter) is designed as an add-on to an existing speaker. They are ribbon super tweeters that have an adjustable cross over with two settings, 11kHz to 15kHz, and 4 settings for sensitivity, 87.5dB-89dB-90.5dB-92dB. They are about $350-400/pair and available from Onix distributors in North America. LCY-100K is another one and costs about $550-600; Madisound and e-speakers no longer carry it. Townshend supertweeter is a third choice at around $1500/pair. There are others.
