You have two options.
1] Import the topics now that you can get at them. You cannot
import a TOC or Index but if you examine those files in Notepad,
you could probably get the content merged via the backdoor.
2] Used merged help but note my method is about merged
webhelp, not merged CHM help.
Help others by clicking Correct Answer if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.