Wouldn't the word just be 'defeminize'? It's in the dictionary right their, either divestment or robbing of feminine qualities. Been around as a word since the late 1700s.

EDIT: also as mentioned, emasculate originally meant 'castrate', and we've just used it as a figure of speech so much that it means the same as 'feminize'.

If we want a word for giving someone a penis, I'm sure I we can come up with it easily. we could make it sound old-fashioned like 'enphallize' or something, or we could be modern and worldly and use something like 'futanize'