If your intent is to define custom users, you can use one of two options to mass-import already defined users:
Store them in a DBMS table and configure a Custom ODBC connection to the database table.
Manually create a single custom user, locate the XML file that stores the details of this user and add the others using the same XML-layout. Usually, this file is called CustomDirectoryData.xml and can be found in C\ProgramData\QlikTech\DirectoryServiceConnector\. But if I'm not mistaken, it isn't present by default.
IMHO in QMC there is no button available to mass-import users from say an Excel file.