Expand internal SSD HD

BootCamp (Windows OS) currently needs to run from the boot drive. So you could alter the boot drive to be the external I don't think you need to go that far here.

You can run apps (Mac or Windows) from a second drive as long as the installer allows it (most do).

You may want to move things around to your external drive so you have more free space on the internal boot drive. Remember Mac & Windows leverage disk for extending RAM (virtual RAM) and page files. I would move all my data to the external.

What if I want to use the external as my Windows drive and leave my Mac internal as my MacOS drive it that possible? Yes. You can make the external a bootable drive and then switch which drive to boot up from using the Startup Manager (Option key).

Here's a good reference: Startup key combinations for Mac

OK, the easiest way is to prep up the external drive as you would for a Mac. Run Disk Utilities to setup the drive as GUID, then run the MacOS Sierra (I wouldn't use High Sierra). Once the drive has the OS on it reboot the system so the external is now booting the system and then configure BootCamp and install Windows. At this point you'll be able to either boot up under either the internal or external drives under MacOS and the with the external boot up Windows under BootCamp.