I believe there is a way to do this in a transfer function with the nrp.leaky_integrator_alpha device type, e.g.
@nrp.MapSpikeSink("left_wheel_neuron", nrp.brain.actors, nrp.leaky_integrator_alpha)
but I believe it is a new neuron that is created and connected to the specified source. So it wouldn't be your readout neuron directly and you would have to properly configure the connectivity/neuron parameters. I believe there are ways to paramaterize that neuron and the connection parameters, but I am not familiar enough with that to help you, sorry.
As far as I know it is currently not possible to directly get the voltage or any other values other than spikes directly from a neuron using NRP provided interfaces.
@georg.hinkel should be able to help you when he is back in the office. There are other potential workarounds for this issue to directly access the neurons, but I will reserve comment until we hear back from Georg.