I've looked into this quite a fair bit as well and in short - not the way you want it to work.
The only option you have is to do a whole HDD back up in order to restore things but this obviously won't allow you to selectively install what you want. If you want to delete content, you can only reinstall it by downloading it from PSN again.
Simply put, the games industry overall seems to think we all have massive internet quotas or everyone has sufficient hard drive space.
My only advice is that you stick a larger HDD in your PS3 if that's an option.