Use custom tags:
http://musicbee.wikia.com/wiki/Custom_TagsNote the wiki is pretty barebones, so here are the steps to create a custom tag in Musicbee containing specific tag information.
1. Create a custom tag for COUNTRY under CTRL+O (preferences) > Tags (1) > Define New Tags
2. Identifier: Name of your custom tag, here "COUNTRY"
3. The rest of the columns on the right correspond how "COUNTRY" is presented in other tag formats. Use Tag Inspector and paste what's in the "Tag Code" column corresponding to the value you wish to add to Musicbee's database.
4. Press "Save" and head back the Tags (1) menu.
5. Under "custom tags", write what you want to call your custom tag under "display name," say "MY_FIRST_CUSTOM_TAG" as an example
6. Under "save to music file as tag," pick the custom tag you just made earlier, here "COUNTRY".
7. Press apply and save. Now Musicbee will rescan your library for the data in the fields you specified in step 3.
Now, you can bring up your country when choosing "MY_FIRST_CUSTOM_TAG" under the list of available tags or for use in Musicbee functions. I used "MY_FIRST_CUSTOM_TAG" as an illustrative example. Feel free to name it whatever you like.