No I don't believe it is possible to add metrics into the UI. The metrics available in MLOps from the dropdown menu are the same as those supported by the Accuracy tab (Accuracy Available in MLOps ), and the Accuracy tab does not support custom metrics (Accuracy tab).
Though I'm sure you could use the API to use your own custom metrics to trigger some action.
import datarobot as dr
project = dr.Project.get('5cc899abc191a20104ff446a')
model = project.get_models()
deployment = Deployment.get(deployment_id='5c939e08962d741e34f609f0')
>>> ('5c0a979859b00004ba52e431', 'Decision Tree Classifier (Gini)')
#Calculate custom metric