Xenko accesses physical files through virtual file system abstraction. So in order to access the asset database, you first need to mount the physical directory into the virtual file system.
Once that is done you can instanciate a new content manager by providing it a virtual path to the asset database.
// Mount the data directory as '/additionalAssets/' virtual path.
new FileSystemProvider("/additionalAssets/", pathToDataDir);
// Create the new content manager
var additionalContet =
new ContentManager(
new DatabaseFileProviderService(
new DatabaseFileProvider(
// The 'db' directory and the 'index' file are now accessible through
// a virtual path.
new Xenko.Core.Storage.ObjectDatabase("/additionalAssets/db", "index"))));