How to add an already existing coordinate system to your favorites?

Or is there a way to export a GCS? 

In the past MicroStation would only open a single GCS file (dty).  Fortunately, it will now open multiple files.  The configuration variable MS_GEOCOORDINATE_USERLIBRARIES defines the location for custom GCS systems.

So you can create your custom GCS dictionaries and store them where they won't be overwritten.

Thanks Ray. Not exactly what i am looking for. I have a DGN with a GCS defined. I am asking, is there a way to export this GCS in a format that will let me add it to my GCS Library without having to rebuild the GCS in question from scratch. I just got a file a client with a weird GCS and i want to save that GCS somewhere other than just in that DGN and import it from that DGN when i need it. It seems like if i want to create a library of custom coordinate systems i need to keep a folder with all of the DGNs in it and them import them when needed. Seems messy. Would love to save them in my favorites or in my regular library. 

Your best option would be to create a seed file containing that GCS and use that file for any new files, however for existing files you can utilize the key-in geocoordinate select reference to copy the GCS from a reference.  That should also work to get that GCS into that seed file.