I'm pretty sure you have to start by setting Primary Text Frames when you create the document. I don't know of a way to go back and change it.
I don't think that's correct. If you draw a new text frame on a master page, then select it, you can click the icon below the In Port in the upper left corner of the frame to make it a primary text frame.
I don't see that as helping with existing frames on the document pages, but it would establish new primary frames for new use.