I checked out the repo. I created a local "macosx" directory in the repo. Then, I created a new XCode project within that macosx directory. To my project, I added the OSS code from the parent directory. Then, I Exported my macosx directory back to the OSS repo. Note, this requires write access to the server that has the repo!
Next, I re-checked out the full repo (to a new directory) and opened the XCode project in it. Now, I am able to select the CVS repo from the SCM Repository list. Woohoo!
But wait! My modifications to the OSS code are not being tracked by SCM! Only my XCode project files seem to be tracked. (When I edit any code, SCM does not tag the files as being modified. Updates from other people are also not being seen). For example, if I right-click on the OSS code, I see "Add to Repository" as an option, while my *_Prefix.pch (obviously checked-in by me) does not have this option (and I can track its changes).
Do I have to add all the files to the repository, although they are already there? This makes me nervous that I might screwup the repo.