Changing the drop down is not currently supported.
But, if your font is supported by all common browsers; or if you already downloaded this font to the end user's browser, you can simply switch to source view in the Iridize editor and change it. like so:
<p style="font-family: "Times New Roman", Times, serif;">text in Times New Roman</p>
text in Times New Roman
If you want to globally change the font of all text in your guides download our design kit from the theme page.