Uwe,
There are several sites and directions that can be used depending on your preferences and capabilities. Just go to google and search 'x509 to pem converter'.
You can view the current CA store from the Web UI. Just go to Network -> Certificates -> Click 'Edit Settings' in the lower panel. Once the setting panel opens in the lower right corner there is a hyperlink 'View System Certificate Authorities', this will open a window and display CA certificate information.
Good luck
Don