Ordinarily OS X needs to create a "boot camp" partition, a process that also involves installing some drivers to Windows so that it'll run properly on the Mac hardware. You can probably install the Boot Camp drivers to Windows separately, but I'm not sure how you'd set up a selector for the operating systems... and to make matters worse, Boot Camp expects that the Windows partition is on the internal drive. Some people can get around this by using a hard drive with Thunderbolt, and I've seen some guides stating that it's possible to do this with a USB 3.0 drive, but don't know of anything definitive.
I have a suggestion for you. I was in the same situation when I switched over close to a decade ago. In order to transition to OS X at your own pace and still use your computer for work, I'd recommend virtualizing Windows. If you're not familiar with it, virtualization using apps like Parallels or VMWare Fusion allows you to run Windows as a "virtual machine" - that is, it's like Windows becomes a program on your computer. Windows behaves exactly as you'd expect it to, with the only possible exception being gaming and graphical performance. The benefit of virtualization over booting into Windows directly is that you can have OS X and Windows running at the same time. You can be in OS X, trying to do something, and then switch over to Windows if you're running out of time or just want to get it done, and then return to OS X... all without having to reboot and reopen programs. When I started out I was in Parallels almost all of the time that I was using my Mac, and with each week I spent less time in it. It was a nice way to ease the transition.
The other nice feature of virtual machines is the ability to turn a physical machine into a virtual one. Both Parallels and VMWare have tools that can be downloaded that will copy your computer (the operating system, documents, and programs) and output it as a virtual machine file that you can then use with the virtualization program. That way you'll be able to access your old computer at any time from your Mac.
Regarding Parallels and VMWare Fusion, each program used to have strengths over the other but these days they're about equal. Parallels is more aggressive about releasing a new version each year and phasing out compatibility with new Windows versions and/or OS X versions; VMWare Fusion used to be better about keeping versions supported for longer, and not releasing versions quite as quickly, but lately it seems as if they've caught on to Parallels' model. Both platforms offer discounted pricing if you're using the competing program, meaning that you can switch over to the competitor for about the same cost as if you were upgrading. I used Parallels, then went to VMWare Fusion, and now I'm back with Parallels... if you have some specific, unusual needs then it might be worth looking into the differences a bit more. For simply running Windows and doing general computing tasks, they'll both get the job done.