I am afraid that this is not possible with Video Studio.
You would need an editing package that supports multiple video layers (eg Premiere, Media Studio Pro etc), or a Video Compositor Tool (eg Video Paint).
In the case of the formaer you could simply sumperimpose a gray clok over the number plate. In the case of the latter you have control over any part of the video frame by frame where you could blur, or block, a part of the image (eg Number Plate).
Good luck |