Yes, you can. But you'll have to write the transformer.
The automatic transformation process can only transform between different mimetypes. This is implemented by the ContentTransformationRegistry. However, by injecting any instance of ContentTransformer directly into an action, you can bypass the registry and perform any kind of transformation as an explicit action.
Some articles relating to actions can be found in the WIKI. To see how this is applied to a like-to-like transformer, check out the code and configurations for org.alfresco.repo.action.executer.ImageTransformActionExecuter. This allows transformations from one image to another without needing to change mimetype.
