Scale selection with locked interior line

[rich2005]

As with most things Gimp, there are several ways of increasing the size of the canvas, also a filter to add a border.  Filters -> Decor -> Add Border

   

Set width and colour. Then if you set Delta Value on color to minimum (1) you get a flat border as a new layer. If required then Layer -> Merge Down  I put those guides in just to show the original image size.