There is no sorting function on fields for scripting in MP3tag.
You would have to export the field contents to an external text-file, let loose an external sorting application and then re-import the updated text-file.
Please not that for V1 tags only the pre-defined genres are valid (and are encoded with an id).
Other applications like WMP may display multiple values for some fields (composer, artist) but WMP uses the semi-colon plus blank to separate entries - the entries then appear as single composers (in alphabetical order created by WMP, though, so there is no need to sort in the fields).
iTunes does nothing like that.
If you are lucky enough to find a player that can cope with these entries, please also note that sometimes an alphabetically sorted order does not reflect other requirements as some terms may represent the major entry (e.g. jazz) and the next one (e.g. free) the minor classification.