Mastered For iTunes tag?

Now when you add purchased music from itunes, sometimes it has a "mastered for itunes" tag under the file settings. Is there anyway to add that tag for ALL albums/songs via mp3tag?

I don't know but you can find out:
Take one of these files and load it into MP3tag.
Select it in the files list.
Press Alt-T - this opens the extended tags dialogue.
Check the list of fields in this dialogue where you find the string "mastered ..." and note the name of the field.
If you do not find any field with that text, then it cannot be done.

To get that field into other files:
Load the files,
select them all,
Press Alt-T
In the extended tags dialogue click on the new button.
Enter the field that you have taken down.
In the text field enter the data you want.
Click OK, until all dialogues are closed to apply the modification.

yea, I can't find the field of "mastered" in the extended tags dialogue..guessing this means I can't do it.

Hopefully it's something to be included in the next update?

The unanswered question is: where do you see it?
If it shows up only in iTunes, it could be information that is only stored in the iTunes database.