Please allow importing subdirectories as the main branch

There is a upsteam project i'd like to daily build. It uses cmake and is actually many projects included into a repository, dir seperated.
So i'd like to get the subdir i'm really after and merge my packaging information

However; lp:munt/mt32emu is not a supported launchpad identifier for merging.

So i tried to work around it; by putting the packing first and using nest-part on the main branch.

# bzr-builder format 0.3 deb-version {time}+{date}
lp:~i30817/+junk/munt_packaging
nest-part munt lp:munt mt32emu .

This causes a exception:
 bzr dailydeb --allow-fallback-to-native muntpackage.recipe working-dir/
Building tree.
Retrieving 'lp:~i30817/+junk/munt_packaging' to put at 'working-dir/muntpackage-{time}+{date}'.
bzr: ERROR: Cannot lock LockDir(file:///home/xxxx/NetBeansProjects/munt-packaging/munt/working-dir/muntpackage-%7Btime%7D%2B%7Bdate%7D/.bzr/branch/lock): File exists: u'/home/xxxx/NetBeansProjects/munt-packaging/munt/working-dir/muntpackage-{time}+{date}/.bzr/branch/lock': [Errno 17] File exists: '/home/xxxx/NetBeansProjects/munt-packaging/munt/working-dir/muntpackage-{time}+{date}/.bzr/branch/lock'