It is possible to debug a running system, without loading an application. To do this, you will need a project with a valid application (.axf file), although it will not be downloaded. If the image that the target is running matches the image in the project, then debug symbols will be available.
You then need to modify the launch configuration for the project such that the Attach Only option is set to "True".
For details of how to create/edit launch configurations, please see the FAQ: