1- Create a dropdown field and enter only one country.

2- Save the new field.

3- Download the attached file 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.