Hopefully, you have someone make an add-on for you or you have the file. I know what to do for logon, but not bootscreen (sorry bub).
From what I know, nLite usually takes add-ons. Otherwise, I would just do it myself.
**Let's say I wanted a logon screen from
http://www.wincert.net/forum/index.php?showtopic=927 or
http://www.wincert.net/forum/index.php?showtopic=4237 **
I'll use Darkside. Download and extracted to the disks I386 directory. That's it
Make sure they're nLite compatible. Some aren't, some are. nLite should give you a warning if it isn't recognized. If it isn't, see if you can do it manually.