They can, just you need to setup backup in Dynatrace Managed first and then backup just the specified backup directory where Dynatrace Managed does the backups.
Backing up anything else from Dynatrace Managed directories makes little or no sense at all.
This question cannot be answered by Dynatrace. We do not certify Dynatrace server with Veritas Netbackup. We suggest you ask Veritas if they certify their product with Dynatrace. Dynatrace does NOT certify this configuration.