24
Use -splice:
convert source.jpg -splice 0x10 out.jpg
If you want to add the border only at the bottom, use -gravity as well:
convert source.jpg -gravity south -splice 0x10 out.jpg
Note that the border will be transparent, unless you use -background, too.
See also Cutting and Bordering for more examples.
Use -extent instead:
convert source.jpg -gravity south -extent 100x50 out.jpg
-gravity tell it which direction to move the original image.