Export as a bmp, edit it with psp or photoshop, import it again as a new bif. I'm assuming you have bif editor? (v 23c is the latest I think)

You can add more shields though I'm not exactly sure as to how to do it. I think they have to be added to some available space (the green background) in the bif file. You then have to add it's coordinates and movement in the /textures/men folder and subfolders.