Welcome tot he Forum @wstewart and thank you for reaching out!
Great question! As it stands, it is not possible to import a CSV to nest a task into several projects, it is only possible to import 1 project at a time. You can read more about this tool in the following Guide article: Asana CSV Importer • Asana Product Guide
I would also encourage you to cast your vote and comment on an existing thread we have in the #productfeedback category requesting for this functionality: Adding tasks to multiple projects via CSV importer.
I hope this helps! Please let me know if there is anything else I can help you with!