Importing an index from a [very] old Robohelp project

Folks,

I have a very old Robohelp project (created in the 1990s I think- maybe early naughties) which I have updated to 2026 version; I can compile it to a single help file with a contents tab and search tab. 

However, the index is proving to be a problem. 

The old project has a massive index file which I can open in a text editor (.hhk file) and I can see it is in HTML format. 

If I stumble around, not really understanding what I am doing and put some links in to create an index in RH2026 it creates an output file as an idx file which looks like it is XML and a totally different format. I cannot see where the information that creates that idx file in RH2026 lives.  

I would have hoped that you could take that hhk file from the old Robohelp project and use it to create the index for the RH2026, but having spent days trying this I cannot make it work. 

All the very old web pages are practically the same, just a few tweaks to bring them up to modern HTML standard, but all the human readable text inside it is identical, so we really want to take that original old index and use it in RH2026. It’s a MASSIVE amount of work to create that index file, there surely must be a way to import a 1990s/early 2000s RH index file into an RH2026 project? 

Many thanks.