This is very much doable through RoboHelp scripting.
A RoboHelp script can perform the automated task of traversing each topic in the project, reading the topic title and inserting the topic title as a keyword in the corresponding topic.
A sample script to achieve the same can be downloaded from this link. This script will append the topic title to the list of keywords and will not remove the existing keywords.
This script is not tested. You can test the script on a RoboHelp sample project first. And if it meets your requirements, you may finally use it for your live project.
Mayank