For V2.3 tags it is possible to create user-defined fields with more or less any name - the disadvantage: hardly any player shows them.
To get a first user-defined field, open the extended tags dialogue (Alt-T)
Click on the "new" button.
Enter the field name, perhaps
Enter the value, e.g. NLD
Click OK until all dialogues are closed.
You can now add that field to the tag panel (Tag Panel – Mp3tag Documentation) and/or as column (File List – Mp3tag Documentation) in the file list.
If you want to copy the contents from the field COMMENT to COUNTRY ORIGIN then try it with
Convert>Tag-Tag
Field: COUNTRY ORIGIN (you have to type it in)
Format string: %comment%