May I suggest that if time is limited, this may not be the best time to go down this route? It can be done by importing the HTM topics from each child into the master but you'll have issues with the index keywords not coming across. You could manually copy/paste the contents of the child project .hhk files into the master project to see if that works. What about other things like context sensitive help, remote topic links though? As I said, it can be done but whether you have time to solve any other issues...
I tend to agree with Colum. Importing the topics is the safer way to go. I am just now adding another module's topics to our Knowleedge Base. As for the indexing. we will do that topic by topic as time permits. Note that with the RH full text search, all topics are searchable across what were your multiple projects. I just tested this again yesterday by using a search term that I know was in the new topics for not the old ones. Most of our users are quite comfortable with the text search and tend to not use the index keys very much. We are in a Tech Support environment and using the AIR help output as it provides the community feedback and updating capability that is important here.
Hope my expereinces are helpful