The -pkg option is used to load Doomsday 2 packages (.pack extension) from the command line. The option takes one or more package identifiers as argument. The packages remain loaded even when game plugins are loaded and unloaded at runtime.
One or more package identifiers (note: these are not file paths).
Doomsday locates packages via its virtual file system. As long as the package is placed in a folder visible in the virtual file system, it can be loaded with -pkg. In practice, placing the packages to load in the runtime folder or any subfolder under the runtime folder is a good choice.