If you want to top-up or add ICP utility tokens to an existing neuron, you have to send tokens to the underlying subaccount address associated with the neuron. This can be found on the NNS dapp by selecting the neuron in the Neuron tab.
On Quill, you have to use the "list-neurons" command.
Once the ICP tokens are sent, the neuron’s balance will be refreshed automatically.
Please refer to this forum guide, How to top-up an existing neuron for more information.