Mass update multiple selection field without losing previous selection

I have several multiple selection fields to keep data about our contacts' interests. When we do a seminar we may have to update 100 contacts with a new interest. I think that all mechanisms to mass update a multiple selection field (mass update from a view, spreadsheet import) would erase the previous selection.

Is there any way to mass update a multiple selection field respecting the previous selection, adding new selected options without unselecting the previous?

Thanks.