I have not done this myself so strongly advise on testing it....

alternatively - you can create a project in Rancher add the namespace in question to that project and then you can assign project level role to the user in question

for reference - if you have not seen this already: https://ranchermanager.docs.rancher.com/how-to-guides/new-user-guides/authentication-permiss[…]e-based-access-control-rbac/cluster-and-project-roles