If you go for the @import or the <link> option the only thing you would have to do is add those to the HTML before you send the mail. So change the HtmlBody with
"@import
url('http://fonts.googleapis.com/css?family=Open+Sans');" + $Email/HtmlBody
Regards,
Ronald
To check if Ronald said does work, i dont think EmailConnector module give you option to edit the email HTML header.
The @import statement can only be added in CSS i think which can only be put in the <head> tag of the HTML.
For this can you please check the SendEmailWithTemplate Java Action? in that the processed Email is coming out. If you can edit that, then add the line mentioned by Ronald in the head tag by adding inside the <style> tags. and then check if all is working.
Good luck.