REQUIREMENT:
As the title says — is this possible via CSV?
I tried using the Relationship > Customer & Contact Together but I’m getting an error
The record you are attempting to load has a different type: job from the type specified: customer.
I need to attach the contact in the Project > Relationships tab without having to manually do it in the UI.
ANSWER:
Attaching new contacts to Projects via CSV is not supported. You need a script to do that.