Last tested: Jan 2019
Roles will be overwritten for a user any time that user logs in with [SAML/LDAP]. So while it's technically possible to manually assign some roles while the user is currently logged in, those changes will be wiped each time the user logs in.
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This is no longer true with the Advanced Role Management options introduced in Looker 7.6. If you have Prevent Individual SAML Users from Receiving Direct Roles toggled on, then you will not be able to assign direct roles, and previously assigned roles will be overwritten on next login. If this option is off, Looker admins can assign Looker roles directly to SAML users as if they were configured natively in Looker.