This cannot be done in an easy way - some ideas only:
I assume that you have full access to the application development and setup, the help content (1), the add-on development and setup and the additional help content (2).
A solution depends on your needs and what you are doing with your CHM's e.g. when context-sensitive help is used in your application. So it's difficult to answer.
The hard way if you don't have the old help source files:
- Decompile the existing content to create the original source files.
- Modify existing content (or add new content).
- Recompile the Help project.
I think you know merging information stuff from our Helpwaregroup site at:
HTML Help - Using Merge
Robohelp makes merging help project easier:
RoboHelp: Merging Help Projects
So, I recommend to build a merged CHM (first try one only) e.g for each of your single application base and add-on's with different file names. A installer must copy the new CHM to e.g. the help subfolder of your application when an add-on is installed.
Of course you need to set the HelpNamespace property or whatever code you have to e.g foobar-0.chm or foobar-1.chm or foobar-2.chm depending on the installed add-on. Or try to overwrite the CHM with only one Name foobar.chm by installer.
Edit: For using special functions of helpauthoring with HTMLHelp (HH) like context-sensitive help you need some more steps and knowledge.
Please read from our Helpwaregroup site:
- Merge & Context Help
IDs
- Context Ids in Merged
Help
Try it for your context-sensitive help needs with e.g. two CHM's in your application and add-on environment.