Yes, you can use all weights of any Google font. For now, go to Google fonts
and select your font and desired weights, generate a LINK tag and paste it into your Web Designer doc in Code View (replacing the existing font link tag).
Then to apply any special weight, select your text and in the CSS panel add the font-family and font-weight rules as needed. Sorry, it's a little bit of a manual process and the special weights you want may not be available in the normal Text panel.
Hope that helps,
Google Web Designer team