Hey. Could you post a link to your site? I can't seem to install a left sidebar as part of the default Frontier settings. Unless I'm missing something?
What you want is possible using CSS media queries. Here's a good intro into media queries: http://mobile.smashingmagazine.com/2010/07/19/how-to-use-css3-media-queries-to-create-a-mobile-version-of-your-website/.
To add CSS to your theme, in your dashboard area go to Appearance->Frontier Options and use the editor under the CSS tab.
If you can provide the link to your site then the volunteers on these forums can more specific help.