I have secrets in one namespace added to a Service Account.
If you then ClusterRoleBind this ServiceAccount, will the ServiceAccount then be usable in all namespaces?
No, service accounts are namespaced and can only be automatically used from within one namespace.
You can not use a service account which is contained in namespace from any other namespace. When you give a cluster role to a service account the service account gets permission to do operations defined in the ClusterRole across all namespaces but it's still usable only from the namespace where it's created.