Yes, in Studio 10 and above it is possible to combine 16:9 and 4:3 clips into the same project. However, it is important to note than black bars will be added to the clip that does not match the projects original aspect ratio. By default Studio sets the project format to match the first clip that is added to the project. As an example, if a 16:9 aspect ratio clip is the first video clip added to a project, then the entire project is now in the 16:9 format. This can be changed in the Project Preferences settings to either set the project format based on the first clip (default) or to use a specified project format for all new projects.
If a 4:3 clip is added to a 16:9 project, black bars will be added to the sides. If a 16:9 clip is added to a 4:3 project, black bars will be added to the top and bottom.