custom feilds

ok heres what i would like to do. i need to add a field to gallery2 that allows me (when uploading an image) to have it add a 6 digit number (entered manually) in black (with black background so no one can view, unless you know its there) after the description but before the image itself. i'm using gallery2 and matrix theme. theme is 100% intact, only changes are cosmetic.

For visual please click the following link:

http://www.gunstocks.com/phpsite/modules/gallery2/main.php/v/Takedowns/Mauser/Mauser/08075K.jpg.html

i'm not having problems with gallery itself, i just dont know how to do this.