Yes you can!
Originally, we opted to use http:// protocol by default because it resolves more quickly. Verifying an SSL ceritifcate for each stop in a redirect can add up to 100-400ms to our link's resolution, which usually doubles the time to resolve. We worked very hard to keep this number as low as possible, hence the http:// default.
However, we absolutely do have an SSL certificate for our geni.us domain. You can manually add the 's' to any of your http://geni.us/xxxxx links and it will resolve with no issues.
According to our research, when you use our domain to redirect links, the lack of an SSL certificate on the link itself does not impact your website's SEO or pagerank.