just abut anything can be imported. But if it is worth it is the real question. An ICS file is a calendar file. HTML is a web page, so sure you could probably convert it back into a calendar, but it would be a lot of work. Likewise CSV is the most basic data interchange format available really, being based on text. It might be useful if you were mail merging items in the calendar, but that is about the only practical use I can think of for it. I would not consider any thing but ICS a good archival format for a calendar.