NTSTATUS
RtlAppendUnicodeToString(
  IN  PUNICODE_STRING Destination,
  IN  PCWSTR Source OPTIONAL
  );

Routine Description:

    This routine appends the supplied UNICODE string to an existing
    PUNICODE_STRING.

    It will copy bytes from the Source PSZ to the destination PSTRING up to
    the destinations PUNICODE_STRING->MaximumLength field.

Arguments:

    IN PUNICODE_STRING Destination, - Supplies a pointer to the destination
                            string
    IN PWSTR Source - Supplies the string to append to the destination

Return Value:

    STATUS_SUCCESS - The source string was successfully appended to the
        destination counted string.

    STATUS_BUFFER_TOO_SMALL - The destination string length was not big
        enough to allow the source string to be appended.  The Destination
        string length is not updated.