It might be possible. I think you'd need to subclass and set the tabStops
on the defaultTextFormat.
While it might be possible it would not be advisable. The mx:TextArea component has inbuilt accessibility support that allows focus change when the Tab key is used. Changing this functionality will be very confusing to people who prefer interacting using a Keyboard instead of a Mouse and will make your site / application impossible to be used by people who are visually impaired.
Just something you might want to consider...