Is it really possible to "boost" 6 V DC to above 50 kV? Or even 400 kV?
Is it really possible to “boost” 6 V DC to above 50kV? Or even 400kV?

Of course.  One common example of something similar (although not as extreme as your specs) is using 12 V in a car to make several 10s of kV to fire the spark plugs.

The same concept can be scaled up to make higher output voltages.  It won't be easy to build something with that stepup ratio and output voltage yourself, but the physics is certainly possible.