You cannot import external orders into the CV3 database. We can and have allowed customers to upload basic order information in a text file to the File Library. A script can then be built to search that text file, find a matching record and return a result on the page. This works great for multi-channel orders when the OMS used doesn't have any kind of web module for order tracking. This isn't linked into our database at all, and it's not as detailed as our built in order history.