I found a trick to achieve this here:
Suggestion for Genres
Go to %appdata%\Mp3tag\data and open usrfields.ini
Add this to the file:
value=COMMENT
name=Comment
multiline=1
The numbers in [#..] must be running from 0 upward through the document. "9" is just a example.
The normal comment field will be still there, but it stops working.
EDIT:
@Florian:
More freedom in the tag panel would be great, though. Especially the limtitaion that, if you move the tag panel at the upper instead to the left border, it can show the fields only in three columns (with a fourth for the album cover). I have free space at the right side of the panel which I can't use. Instead, new fields are added below the third column, which I can only use when i use the scroll bar.
Have a look at the screenshot:
