Click the outport of the main story and click inside the first frame of the index. They will now be threaded. You'll probably need to add a start on next page keep option to the first line of the index.
I must be unclear on the concept. I moved my new index into the existing story between the two desired pages. This shows the original story on the left with the text threads going around the inserted Index story:
Now I click the outport of the text frame on the left and alt click into the Index frame. That overwrites the contents of Index.
Not what I want. I actually want to split the text as well, and just thread thru the index frames and then continue with the original story. I did the above as a test case. For my actual problem, copying the index story in story mode, and pasting it over the appropriate text in the original story solved my problem.
Sorry. I misunderstood. You'll need to cut and paste.