Hi Yonadav.Sudman,
>>Is there a way to register/activate Visual Studio Express 2015 on this machine
I am afraid it is not supported. As far as I know, Visual Studio 2015 express needs to be registered online. So if you want to use express version after VS2015, you have to connect online.
You can consider upgrade to use some paid version like VS2015 Enterprise or Professional in offline environment. The paid version support to use key to register.
Best Regards,
Lake Xiao