You can certainly add a charge pump voltage doubler, but its voltage will sag under the 50mA so you won't get 70V. But its worth trying it, and seeing what boost voltage you'd need to get 70V out the charge pump (it'll be over 35V), then if its low enough, you can just connect the feedback to the charge pump output. It only costs a couple of diodes and a couple of caps.
I would add a low value resistor in series with the fly capacitor, as sometimes hooking a charge pump to a boost converter can stop it switching.
Alternatively, coupled inductors (1:1) are easily obtained, and will give you double the voltage output (or rather, half the peak switch node voltage) if you use it in an auto transformer config. You could also try other ratios, but these are harder to find. A 1:2 for example would mean the peak switch voltage is ~23V.