Boot Camp has very specific requirements for your Windows disc. You can't just use an already installed OS on another HDD.
With that said, both VMWare Fusion and Parallels Desktop have options for "P-to-V" conversions, wherein, an existing PC can be transitioned into a virtual machine to run on your Mac.
But once that's done, you can no longer use that other PC without purchasing an additional license of Windows.
If you truly want to dual boot, you'll want to use Boot Camp with a brand new copy of Windows.