NX Add one extra tab to the File New dialog

Is there an NX setting or Environment Variable to add just one extra tab to the File New dialog for one User?

There's no setting in NX or an Environment Variable to add just 1 tab. To add a tab to the File New dialog, you will need to edit the appropriate template PAX file. 

The template pax files for the File New dialog are stored in the "C:\Program Files\Siemens\NXxxxx\UGII\templates folder unless the environment variable UGII_TEMPLATES_DIR is set pointing to a different location. 

If needed, you can define a location to store the customized PAX files, then set the UGII_TEMPLATES_DIR environment variable locally on the Users workstation to point to the new location so that only that User uses these custom pax files.