Matthias:
I took a quick look at this, and the door component just has the 3D door panel, no 2D symbol (the 2D Script of the panel contains an elevation view of the panel). Rather than edit the panel, the 2D Script of the host door library part would have to be modified to change the appearance of the symbol, not an easy solution, given the complexity of the ArchiCAD door library parts.
To open the component (not that it helps in this case), you have a couple options. One is to simply use the
Open Object.. command and navigate to the library folder where the door panel was saved. You can also use the
Open Object by Subtype... command and drill down to Model Element > Custom Component Template > Custom Door Panel to find you panel. You may need to add these commands to your Work Environment to use them.
David