The coactivation of neurons by stimulating their innate inputs induces their mutual synapse innervations, such that these neurons are able to encode, integrate and store mutiple associated signals, i.e., the recruitment of associative memory cells, for information storage.

Best way to potentially charge up those inactive neurons is to use brain stimulation like tES or TMS. As noted above you can aim to target the neurons directly or rather you can target associated neural cliques which will stimulate them by association. Manipulating oscillations is also a way to recruit neurons.

Perhaps convulsants like gabazine or bicuculline would be useful in the (non-specific) activation of inactive neurons.