Ah! I see. If you look at this forum post, in my example.py file, there is example of how to connect the output of a nengo.Node to drive an ensemble to spike at a certain time. However, this approach still is essentially providing an input current to the neurons to drive them to spike. There is currently no way to do otherwise (i.e., making a neuron spike without changing the input current to the neuron) in Nengo.