Scale selection with locked interior line

[Arenwick]

Hello,

I'm pretty new to Gimp and am attempting to create a border on an image. I'd like to expand the border to have more thickness, but want to preserve the interior dimensions. Is there any way to scale the exterior dimensions while having some sort of "lock" on the interior lines of my selection?

Thanks!

[Krikor]

Maybe - Image - Canvas size

[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.