I don't have the launch dates for those spacecraft, but I can tell you how to specify start and end dates for an object. Add one or both lines to the object description:
Aya Reiko wrote:Can we expect changes to the spacecraft entries (like Mir) in a future release to accomodate this feature?
Definitely. In fact, I've already done this on my version of .ssc . . . Since I just release the exe for prerelease 6, no one else has the new file yet.