Not without code (which you can't do in OnDemand)
The closest I can think of - add a field of type "user picker" and then change your notification scheme so that it sends mail to anyone in that field. Not quite what you're asking for, but it will have the same effect on mail, just not add them as a "watcher"
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Any other country that celebrates Midsummer holiday (this friday 22 June)?
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