Some question as a beginner for making user documentation

That file is only understood by Classic versions of RoboHelp. You haven't said which version of RoboHelp you are using.

• Please go to Help > About and include the full version number.
• If you are using 2019 also state whether it is 2019 Classic or 2019 New UI.

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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.

The reverse engineering method on my site converts a CHM back to the structure for a Classic version of RoboHelp. Classic versions would normally be opened by double clicking an XPJ file but could be opened by double clicking an HHP file. Using the HHP would create an XPJ file. With that you would be able to open the project in the new UI versions of RoboHelp.

As Reverse Engineering (grainge.org) does not give you that vital XPJ file, it's much the same as what @D.Desmet has suggested.

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My site www.grainge.org includes many free Authoring and RoboHelp resources that may be of help.

Hi. With 7-zip, you can rightclick the CHM and select 'open archive'. Then you will see folders and files. The original htm files are somewhere in those folders as well as the images, css and other files. Extract the CHM and then import the htm files in a new RoboHelp project.

Further to Jeff's info, if reverse engineering the chm file doesn't work out for you for some reason, RH can import html files. So theoretically you can import your dreamweaver html pages. How smooth the process is will depend a lot on what is in the files, and you might have to do a lot of cleanup once the files are in RH. I've never done it so couldn't say, but as you're outputting to chm I imagine the files are using relatively simple html and css which shouldn't cause too many problems.  But your mileage may vary. 🙂

Once imported I assume you would also have to recreate the table of contents, index entries, etc. 

Not directly - those CHM files are output and RH project files are not the same. What you'd want to do is called "reverse engineering" - taking the output CHM and creating RH topics out of them - have a look at Peter Grainge's excellent RH resource site - grainge.org - for info on that process.
Another route could be done by using Dreamweaver and outputting something that could be imported into RH directly (maybe Word file(s) - I don't know Dreamweaver much at all).