@alex
I have added the tweeters of the top hifi to my basic hifi exactly the way you describe it - they include a prewired 6db filter (capacitor) to cut off the low frequencies and a plug that fits perfectly into the existing door speaker's receptacle. The filter will keep the impedance almost where it is without the tweeter - so I cannot believe that this configuration will do any harm to your radio.
The sound quality increase - even without additional amp - is remarkable
.
If you need installing information: Look
here (also included: Amp installation advice - thanks to EdP!)
BTW: Forget about upgrading tweeters to the rear speakers without an additional amp - you still won't hear anything
.
Best regards
Mick