How to convert Lead's country field from Text to Pick List

How to convert Lead's country field from Text to Pick List

Hi,
I would like to change the default country field in ZCRM from text to pick list. It looks like not I can't delete default country field and recreate it as pick list nor can i create an new custom field country because such a label belong to default field.
So what do I have to do?
Any ideas?

L