Is there a format for importing multiple choice fields by csv?
I just revisited this issue. As it turns out there's an undocumented feature that allows you to import multiple 'multi-choice' options through csv.
If you list the options separated by comma then they will all show up in the relevant field. Of course your csv editor will have to enclose the field in double-quotes.
There is not, but you can use the quick edit link when editing the field and then copy and paste from your csv file, as long as each option is on a different line.