This program is part of Netpbm.
pnmmargin adds a border around a PNM image.
You can specify the border color with the -white, -black, and -color options. If no color is specified, the program makes a guess.
To remove a border of a specified size from an image, use pamcut. pnmcrop also removes borders, but determines by itself what is border and what is subject.
For lower level control, including to add different size borders to different sides of the image, look at pnmcat.
If all you're trying to do is get the image up to a certain required size, pamcut may be what you want.