Add a solid border without changing the overall size

[rich2005]

Using standard Gimp tools.

border around: 
Add a layer under the icon
Color select the icon
Grow the selection x pixels
Fill layer under icon
Turn selection off
merge layers

border inside:
Toggle alpha lock on
Color select icon
Shrink the selection x pixels
Invert the selection
Fill the selection
Turn selection off
Toggle alpha lock off

Example: 2 minutes https://youtu.be/YbS9dFhGOoQ