Is it possible to increase the capacity of human brain like “Lucy”?
This may provide you with extra supplementary information regarding the notorious "10 Percent Myth". 
To respond to your statement regarding increasing brain capacity through am increase in knowledge:


  Synaptic Consolidation is the process by which items are transferred from short-term to long-term memory. Within the first minutes or hours after acquisition, the engram (memory trace) is encoded within synapses, becoming resistant (though not immune) to interference from outside sources. 


Knowledge has to do with "saving" the original memory traces followed when first learning the information. It does not necessarily have to do with active brain capacity. Thus, an increase in knowledge will not necessarily cause an increase in used brain capacity. 

You see, the primary issue here is energy cost. Although the brain typically only constitutes 2 percent of the body's weight, it utilizes around 20 percent of the body's energy intake. As a species, we have been "designed" through the filtering of many "versions" our species to be most efficient. As we have not found a particular need to utilize such a large percentage of our brain at a time throughout the millennia, our brains have been simplified into a somewhat energy-efficient structure, utilizing a portion of the total capacity at a time. However, if Man had originally needed to use more of the brain capacity for a majority of his or her daily activities, it would be safe to expect that we would use a larger portion of our brains at any given instance. Assuming you are still in a state of heightened neuroplasticity, it is potentially possible to "increase the amount of brain capacity used at a time" by participating in activities requiring multiple parts of your brain. 

But how many different activities can you think of that would require reasoning, planning, parts of speech, movement, emotions, problem solving, movement, orientation, recognition, perception of stimuli, visual processing, perception, recognition of auditory stimuli, memory, and speech?