Is there a way to assign custom metrics to an object on Orion Atlas?

Not directly, you can call in variables for any column on the interfaces table in the database.  One trick that you COULD do is to create a custom property, and write an alert rule that would update that custom property with the data for your custom metric.  Or if you are comfortable in SQL directly you could write a trigger or similar to update the interface custom property whenever the metric is polled (I'm assuming this would be from something like a UNDP)