Library /sys$common/syshlp/uafhelp.hlb ADD, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 newusername

    Specifies the name of the user record to be included in the
    SYSUAF. The newusername parameter is a string of 1 to 12
    alphanumeric characters and can contain underscores. Although
    dollar signs are permitted, they are usually reserved for system
    names.

    Avoid using fully numeric user names (for example, 89560312). A
    fully numeric user name cannot receive a corresponding identifier
    because fully numeric identifiers are not permitted.