Hello Arjaan Den Ouden,
Thank you for your idea. It sounds like you are suggesting a Source field for Bugs, similar to the one that currently exists on the Idea, Feedback, and Feature Request items. I have logged this suggestion internally.
Alternatively, some teams flow all external input through the Idea, Feedback, and Feature Request items. In this way, the Source field could be used to track the external identifier. When teams work in this manner, external inputs will be reviewed and, if determined valid, they are then created as a new Bug, Risk, Issue, or Story and linked to the source Idea, Feedback, or Feature Request item.
Our upcoming Feedback Portal will provide further support for this flow by allowing external customers to interface directly with Feedback, Idea, and Feature Request items without requiring a TeamPulse license. For more information on the Feedback Portal please visit our information site at http://www.telerik.com/agile-project-management-tools/tour/feedback-portal.aspx
the Telerik team