Adding a third node to an existing cluster from a different subnet

Hi

We have an existing MS 2012R2 2 node cluster supporting a SQL Always On setup.    The existing two nodes sit in the same subnet as part of a standard cluster setup.  We want to add a third node to the cluster from a different network to provide HA.  I am trying to understand if it is possible to convert the existing cluster to a multi subnet cluster without an outage?

Thanks in advance