Greenhorn question: Beware!

"Is a 4 ohm exciter speaker and a 6 ohm woofer box, wired in series (total 10 ohm) connected to an 8 ohm output amp OK? "

It’s "OK" in the sense that, AS LONG AS as you don’t push your amp into clipping and/or push the speaker or exciter beyond their comfort zones, I don’t think anything will blow up.

That being said, to the extent that the exciter’s impedance curve differs from that of the speaker, its presence in series with the speaker will change the way the speaker sounds. I expect that the exciter’s impedance rises significantly as we go up in frequency, so I expect that your speaker will have a lot less top end when the exciter is connected in series with it.

So I don’t see any reason why you can’t start playing music through them, but it may not sound very good.

"Is there a capacitor or inductor that could be added in my case, to bring the final impedance into a normal range?"

Imo not without measurements could an impedance equalization circuit be designed that results in the speaker sounding good with the exciter in series.

EDIT:  Typo in the second paragraph of my first post above.  Here is how the sentence should read, caps lock on for the corrected word: 

"The BL would stay the same while Re (effective resistance) would increase, and the net result would be a corresponding INCREASE in Qes."

Duke

Sorry, sometimes my inner nerd takes control.  Lemme try that last post again, this time in English:

You probably won't blow anything up by running the exciters in series with the speakers, as long as you don't turn the volume control up too high.  But the speakers may not sound very good.  I think the highs will be diminished by having the exciters in series. 

I flunked out of mechanical engineering too. 

Duke