Hi, Cédric,
I'd recommend that you download FAR and try adding the
Include statements to your .hhc files with that. FAR is available
from this address:
http://helpware.net/FAR/index.html
After you've started FAR, select Authoring > TOC &
Index Editor, open the required .hhc file, and then select TOC >
Include File to add the required slave TOC at the appropriate
place.
I believe that RoboHelp may suffer from the same bug as
Microsoft's HTML Help Workshop: both tools appear to write the
Include statements at the wrong level in the TOC hierarchy. See the
"Known Bugs" section of this page:
http://helpware.net/htmlhelp/how_to_merge.htm
FAR is free to use for two months, but you must pay $49 or
thereabouts for a licence if you want to use it beyond that.
Pete