Would it be satisfactory to you if you set up a job to automatically re-run the Network Sonar discovery process at regular intervals? I realize this doesn't automatically discover when a new volume is added to your environment; it would be a work around.
I'd think there'd be a way to receive a trap or syslog notification from your server that indicates a new volume is available, and that SW could act on that notification to automatically add that volume.
There ARE alert-based actions that can be created--I use them regularly, just not for discoveries. I think you're onto something interesting, and I'll be watching to see if others come up with a solution to your challenge.
If you DON'T find a way to accomplish your goal, then do two things:
1. Open a Support case with Solarwinds. Their experts will know how to do it, if it can be done.
2. Open a Feature Request here in Thwack and define exactly what you want. Then vote for your idea. Others will see it and vote for it if they believe that it's a good one. After some time and after enough positive votes are received, Thwack and SW admins will analyze your Feature Request and determine if it's feasible. You might find your idea is closed down as not in the future product path, but you MIGHT learn your idea was accepted and will be added to future versions of Solarwinds products.