1- Create a dropdown field and enter only one country.
2- Save the new field.
3- Download the attached filed and extract it.
4- Open phpMyAdmin.
5- Browse the table #__k2_extra_fields and edit the record of the dropdown field created in step 1.
6- Copy the contents of the extracted file and paste it in the value field.
7- Save.
Now you have all countries entered in the dropdown list.