Not automatically, but you can send iCalendar formatted files to the client so that it can be saved to the calendar. I have gotten it to work for the most part, but there is a bug in Outlook 2000 solved in office sp1 I think where it sometimes get's the time wrong from the ical file. Seems to happen when i try to do an all day event, as need to add code for daylight savings etc. Anyways I don't have the code with me here but I do have it at home, still not perfected but I could send it to you if you wish. jsut post you email, or some email you use to trap spam etc and I'll try to remember to send it tonight.
I assumed that you were dealing with outlook clients not connected to an exchange server. However if this is not the case then given the correct permissions (may need a com object to do this) you could alter their schedule within the exchange server itself, just a bunch of databases so probably pretty easy to do. Just a thought anyways.