It would depend on what you have to begin with in order to extract the borders of. Presumably you have the borders as lines whether in inkscape as vectors or as pixel lines in Gimp. Do you want to upload an image of what you have ?
My guess tho is to use a magic wand type selection tool to find the outside of the shape and then expand the selection and then use a fill inside the selection.
Another option is to use an 'erode' type tool which takes a thin line and makes it thicker. If thats not an option then you can usually blur it and use contrast tools to darken the line and up the contrast which also makes it thicker.