How to import multiple values in a text field with Feeds module?
Feeds Tamper will do this for you.


  Feeds Tamper provides a small plugin architecture for Feeds to modify
  data before it gets saved. Several plugins are available by default
  and are described in the examples section below. Additional plugins
  can be added in separate modules or through the issue queue.


One of the default plugins is explode.  Just set up a feed tamper to take your multivalued field from your CSV and explode it based on whatever delimiter you fancy.  This will import it as multiple field values.