I have a pair of Blaupunkt 5.25" 2-ways up front in my Ford Ranger, and
love how they sound with my new head unit, but as I'm sure you're all
aware, you just can't stop tinkering....

What I'm wondering is, if I get a pair of aftermarket tweeters with
crossovers, is it possible to hook them up to my existing speakers and
turn them into components? They would be running off the amp in the head
unit, and at 26 watts/channel I feel that should be plenty. Putting the
speakers in originally was a 5-minute job with the kit from Crutchfield.
How would I go about hooking a pair of tweeters up to the Crutchfield
prefab wiring harness? Is this a fool's errand? Should I just go pick up
a pair of Blaupunkt component speakers off EBay and give away my 2-ways?

Well, there's two schools of thought, here. The first is "what's right" and
the second is "what sounds good".

The "what's right" school would say to ditch your 5.25 two-ways and go with
true seperates. The reason being if you add tweeters to your current setup,
you will basically have two sets of tweeters producing the highs. This is a
no-no in the car audio world as it interferes with imaging and staging and
all that kind of important stuff.

Of course, the other school says "go for it" and see if it sounds good. I
don't believe that it would harm your head-unit, though you would need to
install some passive X-overs on the tweeters if they do not come with any.
A simple capacitor will give you a 6 dB per octive high-pass crossover.
Radio Shack can help you with this.

Anyway, the choice is yours.

MOSFET