Are you talking about publishing or something else? I'm not sure what WYSIWYG has to do with this.
As far as I know, RH doesn't support publishing over SFTP. But you can always use another tool to publish the output over sftp.
For serving the content to your users, you can use HTTPS. (But there has been issues on the forum coming from using HTTPS with WebHelp.) RoboHelp creates static HTML and if you set your server to always redirect http to https, it should work (But there may be issues as some people have reported issues on this.)
Kind regards,
Willam