Try the list of available tools at http://www.pendrivelinux.com
that are able to build a multiboot
You will find several tools running under Windows and/or Linux, and both types of tools can include (merge) UBCD, several Linux distros and more.
Some of the tools are not multiboot, and some of the tools are capable of building either a new UFD or an ISO image for optical media.
The "best" tool for you may depend on the specific distros you are interested in. In some cases, a tool may say it supports a certain distro, but then when you actually test it, the distro (or UBCD, or some other additional option) fails to work (meaning, the distro may seem to boot, but then you can't successfully use it).
I know SARDU (also listed in those lists) works successfully when you want to merge an updated version of PartedMagic with UBCD, among others.
Try them, test them, compare them... Several choices are available.