Yes, you can move/drag any object in FAB 3000. However the easiest way to back off the ground plane layer would be to use "composites". Composites allow you to define clear areas without affecting the existing objects in your Gerber files. Before proceeding here's more information about composites, so you may understand exactly what we are attempting to achieve. https://numericalinnovations.freshdesk.com/support/discussions/topics/14000002372
Here's a quick tutorial showing me scale back the entire plane layer by using the existing board outline, expand the width, assign a "Clear" composite value, and copy that object over to both plane layers.
Note: In this tutorial I randomly chose a composite value of "179". This really doesn't matter however I would recommend using "9" or higher for composite level. I also create an Aperture with the width of 150 mils to use as the width of the composite line. This was an over-dramatic size however I wanted to make it more visually clear for the movie tutorial.
Here's a second tutorial showing how to selectively remove metal areas of the ground by adding a Polygon with a "Clear" composite value.
Note: Any object type (Polygon, Trace, Flash, etc.) that has a composite value can be used to remove any area.
There are several other ways to achieve the remove of metal areas on the ground plane, however the composite method was the first two that came to mind.