Even in a non-design managed environment, when you add a new library, you don't have the choice to add the library definition into an included cds.lib-like file; it adds it into the cds.lib (or whatever file is "forced" with ddSetForcedLib, or via -cdslib on the command line - although I'd advise against doing this because there are almost certainly assumptions in some parts of the tool that the file is called cds.lib).
So I'm not quite sure why you decided on your methodology.
I'm pretty certain that many DM systems recommend that you do it the way that Thomas suggested. Having a quick scan through the SOS documentation, it doesn't seem to suggest that - mainly because they support managing the cds.lib. Historically though I've always done it by managing a project.lib type file - and then manually moving the library into the project.lib when you want it managed.