git-dpm import-dsc --branch branchname package.dsc
will import a .dsc file and try to import it as git-dpm project. If branchname does already exist, it will be added as git parent commit, otherwise a new branch will be created.
For a list of all possible options and their description take a look at the manpage git-dpm(1).
Some important options are:
Include upstream's history by adding it with -p to the parents of the imported .orig.tar file. For example if the git repository you are in contains a tag foo-1.1 you can do:
git-dpm import-dsc --branch foo-debian -p foo-1.1 ../foo_1.1-1.dsc
To be continued...