You are certainly able to import any HTML file into RoboHelp. But the problem is, if these are output files they may well contain additional coding and whatnot that you don't really want to repurpose as source files.
Are you able to show us perhaps the HEAD area of one of these files? There may be telltale clues as to what was used to create them.
You may also want to review my page at the link below where I advise how to re-use output files and make them source files again.
Helpful and Handy Links
Please do be careful of reproducing another company's help. It is their property. They might not worry if it remains internal, but expect crankiness if it gets to the public. One story I heard was a company (Company2) customising the help internally, but it was sent to one of their clients via email and got back to the original company. Company1 was not happy, esp. because the pdf had a "copyright Company2" in it, and sent a cease and desist.