The method of using a combination of cold and hot wallets.

[Sven-TBD]

If the transaction has already been signed by a cold wallet, what you need to do is just submitting the transaction to the node and broadcasting it to miners.
Therefore, you can simply use Neuron's broadcast transaction feature. Just create a temporary wallet in Neuron to enable the 'Offline Sign' button in menu “Tools”. After selecting the signed transaction, the 'Broadcast' button will become available.