Hah ok now I feel like I should add a serious bit to help educate a little here.
Add on tweeters ARE a thing in home audio, but in a different way. A lot of folks have gotten into adding super tweeters to speakers because they want to extend their bandwidth. Call it the HD of audio. It can help make a regular speaker and turn it into an HD speaker.
So is this a thing? Does this need to be done? Does it have merit?
Yes and no, but really, mostly no. There is scant evidence and little of it reliable that suggests we can actually hear the benefits of a super tweeter. There is some good evidence that music recorded using higher sampling rates (and thus supersonic bandwidth) is of some benefit, but we don't know that this is because of supersonic benefits or simply moving problems out of the sonic range.
Let's just pretend like supersonic speakers are of benefit though. Would adding a supertweeter to a regular speaker be good? No I really don't think so. The problem is ensuring good integration. Typically adding tweeters onto regular speakers leads to poor integration in the audible zone. So sure you have a speaker that might have a response extended out to 50khz, but you caused problems at 10khz. 10khz matters a lot more than 50khz.
On top of that, many people do not own systems with a bandwidth out to 50khz, so the tweeters are added for nothing. No benefit.
If you don't like the highs of your speakers, sell them and buy new speakers. I know that is way more expensive than add on tweeters, but add on tweeters are generally a bad idea. you need to really know what you are doing to make that work.
