Yes, you should be able to reenter the HTML Editor instances in PowerCMS in HTML Editor to include your own style sheet. This will allow you to include the various classes you want from the Styles menu in the toolbar.
However, this will not change how the content appears when viewed in PowerCMS.
This is currently just a feature request and something we are looking at handling in a future version of PowerCMS. We have no timeline on when that might be. Sorry.