how to disable "untrusted packages"-warning

[shgeo at online.de (Sebastian Holler)]

I'm trying to build a Debian Etch Live-CD with some packages from an 
additional repository that isn't listed in a gpg-keyring. This causes 
that the build process stops each time with the message "Untrusted 
packages could comprise your system's security ... Do you want to ignore 
this warning and proceed anyway?"

How can I achieve that the build process runs through without this warning?

Sebastian H.