Mysterioso darling,
I do not think Progeny of the Adder was ever filmed. It seems pretty obvious that Jeff Rice, the original author of the story on which Night Stalker was based, must have read the 1965 book. What is quite original in Rice's story is the character of Kolchak--there is nobody at all like him in the Whitten book. In fact, according to the author of the article I read, characterization was not at all Whitten's strong suit.
I checked the internet and it turns out that Jeff Rice launched his own lawsuit against ABC before the movie was finished. It appears to have been settled out of court. The motive for the suit was the fact that the rights to the original story by Rice were sold without his permission. The world is a devious, harsh place.
G.