Add custom attributes to users (or devices) to map hardware peripherals

Yes, this is possible, and I would recommend Microsoft Graph schema extensions or open extensions rather than trying to force everything into Intune custom attributes.

Recommended approach

Keep Intune as your data collection layer. Continue collecting monitor serial numbers, manufacturers, models, and associated device IDs.

Use Microsoft Graph API to update cloud objects with the collected information.

Use Entra ID schema extensions if you want custom attributes attached to device objects.

Consider an external asset database if you need to manage multiple monitors per device, historical assignments, and user-to-peripheral relationships.

Important consideration

Entra ID is not a complete hardware asset management system. A device can have multiple monitors, docking stations, and other peripherals. Storing all this information in a single custom attribute can become difficult to maintain.

My recommendation:

Intune: Hardware discovery.

Microsoft Graph: Automation and synchronization.

External database: Peripheral inventory and assignments.

Entra ID: Store only the relevant reference or summary information.

Documentation: Microsoft Graph extensibility overview .

Bottom line: You don't need to abandon the idea. Graph extensions provide a viable way to associate custom information with Entra objects, but I'd keep the detailed peripheral inventory outside Entra ID.