There is no chance in the current version of Mp3tag.
I have made a proposal to workaround ...
Genre - Standard (Vorgaben) ändern
In your case ...
... use an action to fill the tag field "MEDIA FORMAT" with the list of options ...
#CD
#DVD
#Cassette
#Mp3
... and display this list in an edit field in the tag panel.
When editing this field remove the leading character # from the option to be selected,
save the file,
then run an action to remove the other options having the leading character #.
DD.20140213.1725.CET
If the pick list has not so much items, then you can also use pre-set actions.
For example ...
Format_Media_Format.mta.rar (1.68 KB)
DD.20140213.2122.CET
Format_Media_Format.mta.rar (1.68 KB)