Adding a package from a UNC path requires using an extra \ to the path. e.g:
SFTMIME ADD PACKAGE:"Office2010" /MANIFEST "\\\VBOXSVR\Packages\Office2010\Office2010_manifest.xml" /OVERRIDEURL
"\\\VBOXSVR\Packages\Office2010\Office2010.sft" /GLOBAL /CONSOLE
More here: http://blog.stealthpuppy.com/virtualisation/app-v-errors-00000006-and-000d3002-when-adding-packages/
This forum post is my own opinion and does not necessarily reflect the opinion or view of Microsoft, its employees, or other MVPs.