If they are going in front lower fairings, you will not need to have them FADE front to rear, thus all you need to do is buy some quality (4ea.) 4ohm speakers, wire them in parallel off the existing front two fairing speaker leads, and you will have a 4-speaker 2ohm system that the factory HK radio will see as a normal impedence load, and will create the effect you are looking for.
Honestly though, you are really not doing yourself a favor by just installing 4 speakers up front without an amplifier, as you most likely wont hear the lower speakers well anyway.
Get an $180.00 ELF AUDIO EX2125X amplifier and wire up the 4ea. 4ohm speakers into what will be a 125watt 2ohm system. Look at Ebay.
http://elfaudio.com/mtAmps.html
There are wiring diagrams that show this exact setup in the EX2125X user manual, Page 8:
http://elfaudio.com/images/support/mtAmpManual.pdf
You can buy some 6-1/2 fairing speakers and use 5-1/4 to 6-1/2 adapter rings (ebay also) , and then install whatever size lower fairing speaker pods (5-1/4 or 6-1/2) and again chose the appropriate size speakers.
Use a MARINE rated speaker, there are tons of info in this and other HD forums that people describe their speaker preference.
Save your rear channel of the HK radio for the chance you might install rear Tourpak speakers. But, to make this work, your HD dealer will have to re-flash your HK radio head to allow for FADE .....Front/Rear.
That being said:
Yes, a rear channel (non-FADE) speaker output exists on your factory HK system, you need a wiring diagram to find the correct wire colors.
You "could" do what you originally thought, buy 2ea. 2ohm speakers just for the lowers and run them off the rear channel output of the HK radio, but again, you most likely will never hear them above the engine noise.