Before attempting to merge the partitions you will definitely want to do a backup as others have said. In your case, I would use a software program that actually clones the disks. When I purchased my Samsung 840 Pro SSD, it came with the Samsung SSD Magician software. This software is meant to transfer EVERYTHING that you have on your old disks to the new disk that you just purchased. While this software is free to download from Samsung's site, I do believe that you need to have a Samsung SSD for it to work. So I don't think that this particular software will help you. But you can find disk cloning software out there. For your particular case, you want a disk cloning software.
And as someone suggested earlier, you really do want to run a regular disk backup periodically if you have sensitive data that you do not want to lose. Crap happens when you least suspect it. Hard drives do fail. Without a backup, you run the very real risk of losing your data permanently.
Hope this helps! I will do an online search to see if I can find a free disk cloning program that you can just download and get back with you.
Edit Note: You will need a separate hard drive to do this. An external drive would be great. If you don't have one, it's worth the investment. You can also continue to use it to do your periodic backups as well.