Parent-less branches

Hi,

inside an existing repository I want to create a new branch without a parent commit. Is there a set of commands to do this? Usually only the initial commit of a

The idea is to use that branch to track an external source tarball.  
 From time to time, I would checkout the branch and dump a new  
tarball with fresh source code, commit it to the branch and merge  
again with my master.

I could create the setup this setup if I create a separate repo for each external source tarball and then fetch them into my main repository. But I don't need those repos after that so I'm just wondering if there is a way to create the empty branch directly.

Thanks,