You can achieve this by adding in a custom attribute, Settings -> Manage Custom Properties (Add a new custom property to Nodes , like say "Customer")
1. When you add a node fill this Customer field - specify customer name in here and use the same on your alert
2. if customer name is XXX and node is down - fire an alert
3. I am sure you already have nodes in your environment , you can make a bulk change on them at once and fill in Customer name (use Custom property editor to make the bulk change)