I needed to do a similar thing with my compiled Help and have
documented the steps. I haven't tried it with Web Help but assume
it could work:
1. Create the topic.
2. Select the Project tab and from the File menu, select
Import, HTML File.
3. Right-click on the topic and select Rename. Rename the
file extension to.xhtm. Close the Import dialog without importing
the topic. The topic (in the list of topics) will now have a cross
through the icon as the extension is not .htm but this is OK.
4. Add the topic to the baggage files.
5, Use the map file to map the topic to the correct map ID.
6. Close the RoboHelp project and using Explorer, locate the
project’s ali file.
7. Open the ali file in Notepad. Edit the file extension of
the topic to .xhtm and then close the ali file.
8. Reopen the project, compile and check that the topic is
not found by the search.
Note: It is not possible to open the topic in the WYSIWYG
editor when the file extension has been changed. If you wish to
edit the topic, you must delete the file from the baggage files,
close the project and then use Explorer to change the file
extension back to .htm. Open the project and make the changes to
the topic. When you have made the changes, you must then follow the
procedure above from step 2.
Hope this helps.