Shifted GNDA in Series DC/DC Converter
You're in luck. Start at one end, and call it -200. Now stack 6 +/- 15 V converters. The top of the stack is now -20. If you add both your +/- 5 converters the top of the stack is now at 0, which is what you want. Problem is, your remaining 7 converters will produce a top end of +210. 

Now for the fortunate part: since you do have a spare +/- 5 converter, the change is simple. After you get to 0 volts, add a +/- 5 unit, but only use half of it, to produce +5. Then add your remaining +/- 15 units, but only use half of the top converter. This will add 195 volts to your +5, and the result is +200.

All this, of course, assumes your +/- 15 units really are +/- 15. This is probably not a great idea, particularly under varying loads, although you'll need to check this out. The resulting tolerances may or may not meet your needs.

And I'd check very carefully on the specs for your converters. The number you want is "isolation voltage". If it is not greater than 400 volts you are risking massive failure.