NTSTATUS
RtlAppendUnicodeStringToString(
  IN  PUNICODE_STRING Destination,
  IN  PUNICODE_STRING Source
  );

Routine Description:

    This routine will concatinate two PSTRINGs together.  It will copy
    bytes from the source up to the MaximumLength of the destination.

Arguments:

    IN PSTRING Destination, - Supplies the destination string
    IN PSTRING Source - Supplies the source for the string copy

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.