The following is from Mocha Tosses:
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Subject: Robohelp…
I have a .chm file, but no .mpj. How do I copy what’s in chm file into RoboHelp?
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If you intend to copy legacy projects from older versions of RoboHelp for Word or RoboHelp HTML versions older than X5, the ideal situation is to have the HPJ file if you have a RoboHelp for Word project, or the MPJ if you have a legacy RoboHelp HTML project.
However, our friend Ms. Tosses doesn’t have either. She has an output file (CHM). The good news is that she can use the CHM file to create a RoboHelp HTML X5 project, but the process won’t yield perfect results like an HPJ or MPJ would.
If you would like to learn how to "reverse engineer" your CHM file so it can be used in a RoboHelp HTML project, read this enlightening information: http://www.grainge.org/pages/authoring/reverse_engineering/reverse_engineering.htm