I am trying to add a black border to an image I created with an specific size (98x57mm), how can that be accomplished with krita? I tried several without much luck, for example:7

• I resized the canvas adding one mm on each dimension with a black background, but the black part wasn't homogenous around the drawing
• I tried to draw a black square around the selection but it's very difficult to make it precise.
  

What would be the better way to accomplish this?

I assume that by "homogenous" you mean that the resize did not give equal increase to width and height? If so:

Instead of using mm for the resize operation, can you try using px (with Constrain proportions box unchecked) and add an equal and even number of pixels to the width and height? You won't get exactly 99.00mm and 58.00mm for the final print because the resolution/precision of the mm figure is limited by the dpi you specified for the image.