I still don't fully understand what you want.
Have you already installed the distro's to an external drive?
Does the external drive need to be portable (boot on another PC)? If yes, a live boot environment will be the easiest way (not an installed distro).
Why not setting up your BIOS so it boots from USB first?
Try some bootloaders included in UBCD, to see it they fill your need.
Where is the download for the files needed for a live boot environment to put on an external drive?
Extract the UBCD iso with 7-zip to your USB hard drive.
Install syslinux (\tools\ubcd2usb\) to your USB dr!ve (replace X with the correct drive letter):
syslinux.exe -ma -d syslinux X:
Download Ultimate Boot CD v5.0
Use Parted Magic
for handling all partitioning task: http://partedmagic.com/