Several ways lead to Rome. So there are several ways to do it.
Load (at least) one of these files into mp3tag.
Select it in the file list.
This opens the "Extended Tags" dialogue which lists all tags of the files and allows you to edit them.
If you have to edit these tags frequently then it may be worthwhile to add a column which shows the contents of these special tags. See the Help for this.
Also, you may want to have one of these special tags displayed in the tag panel (on the left of the MP3tag window).
Extend the list of fields in the tag panel via the Options > Tag panel.
If you want to add one the special tags then take the name as shown in the extended tags but enclose this name in %-characters whenever the contents of that tag is referred to, e.g. "Value" and "Field" when adding columns.
E.g. Comment Itunnorm becomes %comment itunnorm%.