Yes, this is possible:
- Copy the attached file track.phtml into the folder app/design/frontend/YOUR_PACKAGE/YOUR_TEMPLATE/template/email/order/shipment/
- Make sure you are using the correct directory for your shop's template / theme.
- To be on the safe side, make a backup of the existing file.
Now all tracking numbers from DHL Versenden will be displayed as links. Numbers from other carriers will be shown as normal text.