As the GUEST mentioned, the idea that our folk melodies have evolved from the Eight Church Modes is speculative--I checked the link above, and it points out that the Greek Church Modes were not universally used:

>The Frankish "Gregorian" chant is not the only form of Western
>Plainchant, although it has since supplanted all but the Milanese
>"Ambrosian" chant, which is still sung in Milan today. Before the
>triumph of "Gregorian" chant, however, there was a variety of Western >chant traditions, including Old Beneventan, Milanese "Ambrosian", Old >Spanish "Mozarabic", Gallican and Old Roman. Of these only the
Frankish "Gregorian" chant adopted the theoretical system of eight modes.

It seems just as reasonable to guess that these other chant traditions could have been sources for our folk melodies--or, even to suppose that folk melodies were used to create the church music in the first place, and the the "theory" was created to describe the music--

Anyway, as to the chords, we know that chord accompaniments were added to folk melodies relatively recently, and that the melodies were often changed from modal to major or minor to make this easier--even still, for Locrian melodies, you can use the II chord as you would a dominant, and resolve it to the VI7--with a B Locrian scale, you'd use C and G7, but end your phrase on the G7--