Adding another image to an existing VMWare application is possible. For adding Windows 10 to VM on a Mac, follow these steps to install the software without a product key.
Download the Windows ISO from IS&T's site : https://ist.mit.edu/windows/10/enterprise
Click on File, then new. You'll get this screen:
Drag the ISO image to the block that says "Install from Disk or image"
Click continue. You'll get the following screen. Fill in the name of the user account you wish to create with a password.
At this point, there is no Product key necessary if you choose the correct installer. Click on the "Choose your Windows version..." dropdown and chose "Windows 10 Enterprise for Virtual Desktops".
On the next screen, click on "Continue Without Key"
Once the OS is installed, log in and ensure you are on MIT Secure. In a few minutes the OS will connect with the KMS server and complete the license process.