OK, try this. In wxMaxima enter the following.
Note the quotes very carefully -- some are single quotes and some are double.
:lisp (setf (get '$mygamma 'wxxmlword) "<g>gamma</g>")
Then (I hope) when you enter mygamma in wxMaxima it
will be displayed a Greek letter gamma.
I don't have wxMaxima installed so I can't try it at the moment, sorry.
HTH
Robert Dodier