I'll attempt to clarify first - If you're asking whether you can apply your styles to the content as you edit text in the FCKEditor, the answer is yes.

The FCKEditor allows you to apply a specific CSS File to the Editor Area. There are a couple ways at it. If you want to apply a CSS file to the FCKEditor across your instance of DNN you can edit the fckconfig.js file and specify a file for the Editor Area. For more information - check out this link.

Of you can apply a file as an administrator to the Portal, the module, or an instance of the module using the "Show Custom Editor Options>Editor area CSS"

Hope this helps.