If you cannot find a more direct way to do it, you could use XBoot to make a top-level ISO list to choose from and have UBCD4WIN as one of the options.
I have used Xboot to make a CD with a whole load of useful bootdiscs in the past, and it worked well.
EDIT: I just noticed that there are instructions for doing this on the UBCD4WIN.org website, so you do not have to use XBoot.