Could an upgrade edition of Windows be used to install boot camp if you connect two external DVD drives?
What are you trying to upgrade it from? If the answer is WinXP, then no.
From Apple's 10.6 bootcamp install pdf:

Upgrading to Windows Vista or Windows 7
You can upgrade Windows XP to Windows Vista or Windows Vista to Windows 7.
Note: If you’re using Windows XP, you cannot upgrade to Windows 7. You can perform
a clean install (also called a custom install), which erases your Boot Camp partition.
Before you perform a clean install, make sure you have the installation discs for all your
programs and a backup of your personal files and settings. You can restore them after
the installation.