In versions of Tableau Server using tabadmin (Tableau Server for Windows 10.5 and earlier) it is possible to configure Named User Delegation for Impala using the command tabadmin manage_global_credentials.
Since we added support for Kerberos delegation for Impala in version 10.0 (see Enable Kerberos Delegation for Hive/Impala) we are not seeing much call for Named User Delegation.
Therefore, we have not initially implemented support for configuring this setting in Tableau Services Manager (TSM). TSM first shipped in Tableau Server for Linux 10.5 and will be coming to a future release of Tableau Server for Windows.
If you are interested in being able to configure Named User Delegation for Impala with TSM, and Kerberos delegation for Impala is not a viable alternative, please vote for this idea to let us know!