Pretty sure you can't select a folder, but you can choose files or folders marked with a condition.
First set a condition on the folder in the global project.
In the import file, click Edit next to Import Conditions, and set your condition to Include. Also select Auto-Exclude Non-Tagged files.
Thanks Dave.
That's a shame. In a big global project that's used by many target projects it would be more intuitive just to use the directory structure (perhaps supplemented by conditions).
But anyway, thanks for your help.