If you want to use CVS with a project that is currently being stored in a version control system other than CVS, you need to convert the files from the other system to CVS. Fortunately, scripts to do just that are available on the Internet and in the CVS source distribution.
The rcs-to-cvs script checks RCS files in to CVS. It also checks in files that didn't previously have source control. It starts by checking whether the current directory's subdirectories exist in the repository; then it commits the files. Files that do not already exist in the repository are checked in as revision 1.1. rcs-to-cvs is available from the contrib directory in the CVS source code.
B.5.2. rcs2sccs and sccs2rcs
These scripts convert between RCS and SCCS version control. Note that rcs2sccs supports only one level of branching. Both scripts retain as much of the file metadata as possible. rcs2sccs and sccs2rcs are available from the contrib directory in the CVS source code.
VSSExtractor converts files stored under Visual Source Safe to files in a CVS repository. At the time of writing, it is still under development and hasn't reached its 1.0 milestone yet. VSSExtractor is available from http://sourceforge.net/projects/vssextractor/.
pvcs2rcs reads files from Merant PVCS version control and writes a copy of those files using RCS version control. It acts recursively and leaves the PVCS archive unchanged. pvcs2rcs is available from the contrib directory in the CVS source code.