I don't know a way with the menu editor. However, you can generate the menu to a .MPR (generate menu item on the Menu pad) and then manually edit the file. At each of the DEFINE PAD and DEFINE BAR statements you would add the following:

[FONT cFontName [, nFontSize [, nFontCharSet]]] [STYLE cFontStyle]

Look at the help file for more.