From the Zoiper skin go to "Add" and select "Import from another force", then choose "Import from CSV".
The first line is a header and describes the structure of a record.
When you are about to import from a CSV file, the fields to be used for populating contact information must be selected from the corresponding drop-down menus. It's best to use the ones in the header.
Afterwards you will see a list of contacts and you can select which to add.
Note that double quotes are needed to escape the following in a field value:
1) line breaks (CRLF)
2) double quotes