A normal plugin is almost useless. You would end up typing the clock commands all the time. And where would it show? I think your contact wouldn't be happy if a time string appears all the time in the conversation so that you can see what time it is in some part of the world.
However, you could say: why not show it in a seperate window. Indeed that would be much better. But then again, you don't need a plugin for this at all. There are already hundreds of (free) programs which you can install which do just the same.
If you realy want to view it inside MSN Messenger itself, the best option you could explore is to add a custom tab with the times of your choice. Either make your own tab (a tab is actually a simple html page, nothing more, so you can easly write some html which shows every clock you want) or sign up for one of those hundreds services where you can pick and choose your clocks (if you don't know html and javascript). Either way, you'll need
TabServe to patch your msn messenger and show a custom tab. So patch your messenger with TabServe, sign up for the TabServe service and point a tab to the URL of your choice (weither it is your own made html page on the web or an existing service)...