"Combine" is probably a poor way to do this.
The simple, straightforward way is this. You mount the old drive physically in your desktop's case in a drive bay and connect to it both power supply (from the PSU) and data cables - the latter connects to one of your mobo's unused HDD ports. Most mobos have at least 4 HDD ports. When you boot up your machine it should boot normally, except that you will find that old HDD showing up in My Computer as a new separate drive with its own letter name, like "F:", or something like that. All your old info on it is fully accessible to you. The old OS on it is of little use to you and you should NOT try to use it. As long as you do not modify your BIOS Setup Boot Priority settings, your machine should not try to boot from the old drive.
There is a way to make the two drives appear to be only one so they can be used that way - you might call this arrangement "combined". However, it might be tricky to set up, and harder to keep things sorted out, than just keeping them on the two different drives. It's like having two sets of files in separate file cabinets.