There is no option in IBM Installation Manager (IM) to import existing installations of packages. If there is some workaround in existence to do this, it is definitely not supported by IBM. You must point IM to a local repository and install RAD and/or RSA from the repository you have created. Alternatively, you can configure IM to search IBM service repositories and pull down information from their online repositories.
Here is a link that shows how to point IM to a local repository.
Here is a video that shows how to install RAD. I found this easier to follow.
Can you maybe provide the post where you read that? I would be curious to take a look at the post!
Hope this helps!