[Zope] override modification_time at manual entry?

I have some old diary entries which I'd like to move into a ZWiki. ZWiki doesn't have any explicit date fields, it just uses the standard modification_time for RecentChanges listings. What would I have to do to make a form where I could create a new ZWiki page and have a datetime field on it which would become the initial modification_time for that object? Am I going to be able to add a field to the form, add a single line of code or so to map the fieldname to the object property? Or is Zope going to object to that property being touched by anything other than its internals?