I have successfully downloaded metadata for one of my apps from iTunes Connect using Transporter with the following command:
iTMSTransporter -m lookupMetadata -u <username> -p <password> -vendor_id myAppSKU -destination ./
This creates a new folder called myAppSKU.itmsp which contains a single file called metadata.xml. Is there any way to force Transporter to download the screenshot assets for my application to the same folder or must I do this by hand?
No, there is no such functionality for the iTMSTransporter. (at least not yet)
Source: The Transporter User Guide and the fact that I've been dealing quite a bit with iTMSTransporter for a project.