Yes, that should be easy. All you need is an 'auto-enter' calculation. but with the 'do not replace existing value of field(if any)' checkbox ticked. This will work with any new ones, you then need to copy the calculation go into the field in browse mode, with all records showing and do a mass-replace (Press COmmand+= on a mac or CTRL+= on windows) with that calculation for your existing data.
--James Chiswell Jones
thanks all, it was Phil's elegant solution that I'd remembered seeing before but couldn't find again