Only a Project member can add observations to a Traditional Project. I don’t think they have to be the member’s own observations.
You could ask one of the Project members to add the observations you want to the project.
I believe there is some related permission, that you have to allow others to add your observations to Projects, but I can’t find it.
Ref: Adding Observations to a Traditional Project - Wiki
P.S.: May I ask why you don’t want to join the Project?