Try though I have, I can find no help on this in Illustrator's Help. Yet this seems to me (and others) such an elemental thing to want to do, yet there does not seem any way to do it except to draw a separate line segment, cut the existing path to create two endpoints, then join the three endpoints. ???!!
Please tell me I am missing something completely.
"I am but an egg."
--Stranger in a Strange Land