Home-made Giant batteries
Booster, if I understand what you would like to do correctly, you wish to build some sort of storage device in which you would store electricity from lightning which you'd stimulate using sounding rockets trailing a wire in order to "jump-start" the arc. This sort of stimulation is done my researchers quite often so I see no major hurdles with that portion of the problem. However there are very large problems with your storage and conversion issues. Let me explain:
Chemical batteries are not suited to this purpose, they tend to explode because they cannot absorb that much energy that fast. They also are voltage limited. A lead acid cell can only absorb about 2.3V per cell. in order to absorb a million volts you'd have to have a half million cells! A Leyden jar (a very large and primitive air gap capacitor) might better suit your purpose but the plate gap insulation would have to be HUGE to be able to withstand the voltage.
Even if the storage problem could be overcome, I cannot imagine that you could build a voltage converter that could step it down to reasonable voltages easily. Remember, it will be DC voltage and you'd have to "chop" it and feed it to an exceedingly well insulated transformer to step it down to to something approaching manageable size. Not many switching devices can handle a million volts.
I'm not saying that in theory, this is not possible, merely exceedingly difficult, and not to mention EXCEEDINGLY dangerous. This would not be something I would suggest you attempt without much more knowledge than we can provide you.