Ok, so basically uou are asking about a sort of Matrix style "human battery" I'm assuming, with your definition of the "soul" being a collection of a lifetime's brain wave activity.
First point - Brains do not work like that. Obviously if you have not done any specific research into brain anatomy, you may not know this, however brains do not use "waves" as such, rather they use impulses. If you do a little more research into this, you'll see what I mean, but the basic rundown is that your brain is made of millions (the average for an adult human being about 100 billion) of nerves or synapses. These transmit and use impulses that travel along them in order to make decisions and thoughts. You brain is doing millions of actions per second, all of them fuelled by the food you consume, and carried out by sending these impulses down the "wires" of your brain. A good analogy for a human brain is like a computer processor, only a LOT more complicated.
So, these impulses are not stored in any way, after they serve their purpose (consisting generally of fractions of milliseconds of moving an electrical impulse from one synapse to another) they simply vanish and never reappear. This is not to say that there is no debris, there is always some form of waste produced, but is of no use.
So in your definition, the "soul" of a human would be useless after they die, as no more impulses take place and the brain is totally inactive. There is no "soul" as in a ghost, these have no scientific grounding currently and are generally considered to not exist.
Tl:dr is this - They wouldn't be able to, a human body is useless after death, they would need them to be alive. Don't worry, no aliens are harvesting us.
Just as a quick afterthought on this - even if the human was alive and thinking at maximum capacity, and the aliens could harvest this electricity, it would be of little use, the actual current made by a synapse going off is miniscule, and very little electricity is produced over hours of brain use.
Hope this helped!