"By merging all these projects into one, there maybe some
topics that will need to be tagged to all 420 .chms."
Sorry, not following you.
"Also, would there be any way of importing the .chm files
from all the projects or would i have to start again with that
one?"
You mean your existing outputs? Why? You have them elsewhere.
When they change you will create new ones from the new project.
Help others by clicking Correct Answer if the question is answered. Found the answer elsewhere? Share it here. "Upvote" is for useful posts.