how can I merge multiple FrameMaker TOCs/IXs into one RH TOC/IX

I am trying to link a single RoboHelp project to mulitple FrameMaker books.  So far so good---from the WebHelp layout, RH generates the files and I can view each book in my browser---but I cannot find a way to merge the individual Frame TOCs and IXs.

I have a User's Guide, an Installation and Adminstration Guide, and a Plugins Guide in Frame.  Each is a separate book, each has its own TOC and IX.

In RoboHelp, I have a redseal_docs project which I have linked to each of the three books.  In the Project Manager, I right click on the User's Guide and open the Properties dialog, where I enable conversion of TOC and IX, creating new RH TOC and IXs named usr_gde.

I generate the book, it looks good, Frame TOC and IX are correct.

Then I right click on one of the other books and enable conversion of TOC and IX, but my only option is to create a new RH TOC and IX.  The Add to existing... options in the dialog remain greyed out and not selectable. 

The end result is that I can have TOC and IX in RH from only one of the three Frame books, when I want one RH TOC and IX to consists of a merge of the three.

Is that reasonable?  Can RH do it? And if so, how?

Thanks.